Optionally embed NOTICE files in apks.

If embed_notices or ALWAYS_EMBED_NOTICES is set, collect NOTICE files
from all dependencies of the android_app, merge them with the app's own
one (if exists), transform it to HTML, gzip it, and put it as an asset
in the final APK output.

Bug: 135460391
Test: app_test.go + Built Mainline modules
Change-Id: I52d92e2fd19b3f5f396100424665c5cc344190d8
Merged-In: I52d92e2fd19b3f5f396100424665c5cc344190d8
(cherry picked from commit 5b425e2e)