Add config.AndroidFirstDeviceTarget.

Targets[Android][0] is not always the first, preferred target. Add a new
field to config to provide the correct target and begin to use it.

Test: m nothing
Test: build/soong/scripts/build-ndk-prebuilts.sh
Fixes: 156980228
Change-Id: Ib56f52424ebf47f3eee6ac62b9fd9eca2919d63a