AdapterService: Move variable to final

These field can be initialized at constructor:
* Looper
* handler
* DatabaseManager
* SilenceDeviceManager

We could probably extend this list in the futur.
But at the moment this is the bar minimal require for passing test when
we enabled fastBindToApp flag

Bug: 323672160
Test: atest BluetoothInstrumentationTests with & without fastBindToApp
Change-Id: I50c2b15c89586cdc15f4d783d249f3cc830b37b4