Since gradle android plugins 2.2-alpha4:
Gradle will attempt to download missing SDK packages that a project depends on
Which
For the new sdkmanager utility:
sdkmanager
yes | $ANDROID_HOME/tools/bin/sdkmanager "build-tools;24.0.3"
There's a bit of a delay between yesses, so the command could hang with the license showing for a while, but it doesn't require human intervention.