Fix Null Pointer Exception

Do not clean the mDatabaseManager when the service stop which align
with other services approach and avoid crash for getting the
connection policy when service is stopped.

Tag: #stability
Bug: 262529287
Test: atest BluetoothInstrumentationTests
Merged-In: Idda86e857108db4cc847e3e2cd0a40ba0ce6ee36
Change-Id: Idda86e857108db4cc847e3e2cd0a40ba0ce6ee36
(cherry picked from commit ccbc6ace)