LeAudio: Improve service cleanup

Some fields were not cleaned up on service shut down.
This was visible in the unit tests.

Bug: 254925603
Tag: #feature
Test: atest BluetoothInstrumentationTests
Change-Id: Ia322147e1d92ddd76f81d95558b3cd4a8e613dbc
Merged-In: Ia322147e1d92ddd76f81d95558b3cd4a8e613dbc
(cherry picked from commit 43dabc4d)