ProfileService: remove adapter and set binder as final

mAdapter is not possible to be null when a service is starting
mBinder should never be set to null as it is not expected to be cleared

Bug: 291815510
Test: m com.android.btservices
Test: atest BluetoothInstrumentationTest
Flag: EXEMPT, no-op
Change-Id: Ibad098f5bc36fe5d1966842325afb039ea6721b7