Convert core/tests/featureflagtests/Android.mk file to Android.bp

Cherrypick of https://googleplex-android-review.git.corp.google.com/c/platform/frameworks/base/+/6924943
See build/soong/README.md for more information.

Bug: 122332340
Test: treehugger
Change-Id: If3f574bc15dedcd7880be605667297d257175c8d
Merged-In: If3f574bc15dedcd7880be605667297d257175c8d