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
Merged-In: I5b2973c12b3d36663fadd2e5ed362602785e1447
Change-Id: I5b2973c12b3d36663fadd2e5ed362602785e1447
(cherry picked from commit 59867e43c84de5563c73043594e13941cd016246)
10 files changed