Move AudioGain to libaudiofoundation.

1. Create folder for libaudiofoudation.
2. Move AudioGain to libaudiofoudation. The goal is to make AudioGain
be able to use in binder call and be vendor-available later.

Bug: 135621476
Test: make, CTS for AudioTrack, AudioRecord, AudioManagerTest
Change-Id: Ibf16ad9cb7f2ac657a02632ccbe4f934b5a12731
26 files changed