AppCompatOverridesService: Remove finalizer and unregistration logic

This has no hope of currently working in production, because
DeviceConfig holds a strong reference to the listener, so the finalizer
never gets called - besides, this is a singleton service in
system_process that shouldn't ever get collected.

During testing, the finalizer is unnecessary for the same reason. The
only way the class can be finalized is if the Context and the
DeviceConfig both are unreachable, but then there's really no need to
unregister anymore.

However, during AppCompatOverridesServiceTest the finalizer invokes
DeviceConfig methods on the finalizer thread concurrently with
non-thread-safe stubbing in TestableDeviceConfig, causing occasional
test flakes (that, due to the combined effect of the bugs at b/399645852
cause every subsequent test in the class to fail).

Bug: 397997489
Test: atest AppCompatOverridesServiceTest
Flag: TEST_ONLY
Change-Id: Ic5ed380286df6cd41c9b1a504e237d320c37080c