Android 7 introduced some changes to the way certificates are handled (http://android-developers.blogspot.com/2016/07/changes-to-trusted-certificate.html) and somehow I cann
The solution is do not use .p12, just navigate with Chrome (with configured proxy on wifi) to http://charlesproxy.com/getssl and install downloaded .pem file.
I had exactly the same problem on my Nexus 5X running Android 7.0. There was previously exported .p12 from Charles 3.11.5 (Help->SSL Proxying->Export Charles Root certificate and Private key). When I tried to install .p12 from phone (Settings->Security->Install from storage) it appears only under "User credentials" and never at "Trusted credentials", and of course SSL with Charles proxy did not work.
The total "how-to" for Android 7.0 would be like that:
android:networkSecurityConfig="@xml/network_security_config" to at Manifest.xmlP.S. Check date/time on the device. It should be correct.