Migrate PlatformLibrary to Soong

Had to make additional changes to fix break from previous version of change.
This includes generating content for api/*.txt files for backwards
compatibility using m upgrade-api as described in an error message.

Fixes: 203691604
Test: jar comparison -- identical
Test: Build sample_addon-eng (which broke before)
Change-Id: Ic054989dfe64019b9b05ee61f0c82e0a0d4aaa3f
10 files changed