Convert libtextclassifier to Android.bp

See build/soong/README.md for more information.

This no longer supports LIBTEXTCLASSIFIER_STRIP_OPTS as a command line
argument, that would require some custom go code to define your own
module type. Instead, I've just left the setting that can be uncommented
/ changed as desired.

Test: cd external/libtextclassifier; mma
Test: atest frameworks/base/core/tests/coretests/src/android/view/textclassifier/TextClassifierTest.java
Change-Id: I65959ec248b04dbaa08edf38dab2edf7d124c1ed