AudioFlinger: Extract inner AudioStreamIn and Source classes

Ensure member variables are const.

Test: atest audiorecord_tests audiotrack_tests audiorouting_tests trackplayerbase_tests audiosystem_tests
Test: atest AudioTrackTest AudioRecordTest
Test: YouTube Camera
Bug: 288339104
Bug: 290269791
Merged-In: Idb7586437ce43ae1c8455bcef645a943ad2e79de
Change-Id: Idb7586437ce43ae1c8455bcef645a943ad2e79de