AudioMixer: Enable 256 tracks, up from 32 tracks

Client apps can have 40 tracks, up from 14 tracks

Related code cleanup:
a) Removed State nested class
b) Moved static functions to Track member functions
c) Moved static function hooks to pointer-to-member-function hooks
d) Some reorganization of touched code for recent C++ style

Test: test-mixer native mixer test
Test: SoloTester effect test
Test: SoundPool with many tracks
Test: CTS AudioTrackTest
Bug: 64161002
Change-Id: I0d09620acd715d577e776bb6f76e94e87e554520