LoudnessEnhancer: Fix gain precision and glitching

Fixes:
1) Remove state reset when changing target gain, eliminating click.
2) Eliminate long term gain precision drift from delta multiplication.
3) Reduce files in preparation to move to audio_utils.

Flag: EXEMPT bugfix
Test: atest loudness_enhancer_tests
Test: Manual with Clarity and Oboetester
Bug: 399924660
Change-Id: I8838e626657fb938c7ecee8b6af27de1be435690