Navigating to http://whatsmyuseragent.com/ shows me my stock Android browser on my Galaxy Nexus running 4.2.1 has the user agent
Mozilla/5.0 (X11; Linux x86_
It looks like the presence of "Linux; U; Android" is a consistent differentiator for the stock browser. Who knows what the "U" means, but I suspect it will be unreliable in the future. Hopefully the browser will either go away or start to play like Chrome when the strings change. Some other strings are below for comparison
Android Chrome: "Mozilla/5.0 (Linux; Android 4.1.2; SAMSUNG-SGH-I727 Build/JZO54K) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/28.0.1500.94 Mobile Safari/537.36"
Android Firefox: "Mozilla/5.0 (Android; Mobile; rv:23.0) Gecko/23.0 Firefox/23.0"
Android Opera: "Mozilla/5.0 (Linux; Android 4.1.2; SAMSUNG-SGH-I727 Build/JZO54K) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/28.0.1500.63 Mobile Safari/537.36 OPR/15.0.1162.61541"
"Android Browser" is the name of a browser. Confusing perhaps, but with a question like this, it shouldn't be. It's the browser that comes with the vast majority of Android devices, and he's obviously not talking about Chrome.
According to http://www.useragentstring.com/ this is the full list of Android Stock Browser strings. I am currently unsure whether there is a better way to select all of them at once, since some will continue to be added, and also which would not accidentally target Chrome on Android. Hopefully someone gives you a better answer as I am curious myself of the best way to go about this.
Mozilla/5.0 (Linux; U; Android 4.0.3; ko-kr; LG-L160L Build/IML74K) AppleWebkit/534.30 (KHTML, like Gecko) Version/4.0 Mobile Safari/534.30
Mozilla/5.0 (Linux; U; Android 4.0.3; de-ch; HTC Sensation Build/IML74K) AppleWebKit/534.30 (KHTML, like Gecko) Version/4.0 Mobile Safari/534.30
Mozilla/5.0 (Linux; U; Android 2.3; en-us) AppleWebKit/999+ (KHTML, like Gecko) Safari/999.9
Mozilla/5.0 (Linux; U; Android 2.3.5; zh-cn; HTC_IncredibleS_S710e Build/GRJ90) AppleWebKit/533.1 (KHTML, like Gecko) Version/4.0 Mobile Safari/533.1
Mozilla/5.0 (Linux; U; Android 2.3.5; en-us; HTC Vision Build/GRI40) AppleWebKit/533.1 (KHTML, like Gecko) Version/4.0 Mobile Safari/533.1
Mozilla/5.0 (Linux; U; Android 2.3.4; fr-fr; HTC Desire Build/GRJ22) AppleWebKit/533.1 (KHTML, like Gecko) Version/4.0 Mobile Safari/533.1
Mozilla/5.0 (Linux; U; Android 2.3.4; en-us; T-Mobile myTouch 3G Slide Build/GRI40) AppleWebKit/533.1 (KHTML, like Gecko) Version/4.0 Mobile Safari/533.1
Mozilla/5.0 (Linux; U; Android 2.3.3; zh-tw; HTC_Pyramid Build/GRI40) AppleWebKit/533.1 (KHTML, like Gecko) Version/4.0 Mobile Safari/533.1
Mozilla/5.0 (Linux; U; Android 2.3.3; zh-tw; HTC_Pyramid Build/GRI40) AppleWebKit/533.1 (KHTML, like Gecko) Version/4.0 Mobile Safari
Mozilla/5.0 (Linux; U; Android 2.3.3; zh-tw; HTC Pyramid Build/GRI40) AppleWebKit/533.1 (KHTML, like Gecko) Version/4.0 Mobile Safari/533.1
Mozilla/5.0 (Linux; U; Android 2.3.3; ko-kr; LG-LU3000 Build/GRI40) AppleWebKit/533.1 (KHTML, like Gecko) Version/4.0 Mobile Safari/533.1
Mozilla/5.0 (Linux; U; Android 2.3.3; en-us; HTC_DesireS_S510e Build/GRI40) AppleWebKit/533.1 (KHTML, like Gecko) Version/4.0 Mobile Safari/533.1
Mozilla/5.0 (Linux; U; Android 2.3.3; en-us; HTC_DesireS_S510e Build/GRI40) AppleWebKit/533.1 (KHTML, like Gecko) Version/4.0 Mobile
Mozilla/5.0 (Linux; U; Android 2.3.3; de-de; HTC Desire Build/GRI40) AppleWebKit/533.1 (KHTML, like Gecko) Version/4.0 Mobile Safari/533.1
Mozilla/5.0 (Linux; U; Android 2.3.3; de-ch; HTC Desire Build/FRF91) AppleWebKit/533.1 (KHTML, like Gecko) Version/4.0 Mobile Safari/533.1
Mozilla/5.0 (Linux; U; Android 2.2; fr-lu; HTC Legend Build/FRF91) AppleWebKit/533.1 (KHTML, like Gecko) Version/4.0 Mobile Safari/533.1
Mozilla/5.0 (Linux; U; Android 2.2; en-sa; HTC_DesireHD_A9191 Build/FRF91) AppleWebKit/533.1 (KHTML, like Gecko) Version/4.0 Mobile Safari/533.1
Mozilla/5.0 (Linux; U; Android 2.2.1; fr-fr; HTC_DesireZ_A7272 Build/FRG83D) AppleWebKit/533.1 (KHTML, like Gecko) Version/4.0 Mobile Safari/533.1
Mozilla/5.0 (Linux; U; Android 2.2.1; en-gb; HTC_DesireZ_A7272 Build/FRG83D) AppleWebKit/533.1 (KHTML, like Gecko) Version/4.0 Mobile Safari/533.1
Mozilla/5.0 (Linux; U; Android 2.2.1; en-ca; LG-P505R Build/FRG83) AppleWebKit/533.1 (KHTML, like Gecko) Version/4.0 Mobile Safari/533.1
Mozilla/5.0 (Linux; U; Android 2.2.1; de-de; HTC_Wildfire_A3333 Build/FRG83D) AppleWebKit/533.1 (KHTML, like Gecko) Version/4.0 Mobile Safari/533.1
Mozilla/5.0 (Linux; U; Android 2.1-update1; es-mx; SonyEricssonE10a Build/2.0.A.0.504) AppleWebKit/530.17 (KHTML, like Gecko) Version/4.0 Mobile Safari/530.17
Mozilla/5.0 (Linux; U; Android 1.6; ar-us; SonyEricssonX10i Build/R2BA026) AppleWebKit/528.5+ (KHTML, like Gecko) Version/3.1.2 Mobile Safari/525.20.1