AudioFlinger: Extract inner Client class

Test: atest audiorecord_tests audiotrack_tests audiorouting_tests trackplayerbase_tests audiosystem_tests
Test: atest AudioTrackTest AudioRecordTest
Test: YouTube and Camera
Bug: 288339104
Bug: 289135349
Change-Id: I5b2973c12b3d36663fadd2e5ed362602785e1447
10 files changed