Add tests to ensure input settings are loaded on boot

The input settings are loaded on systemReady().

To add these tests, we must mock UEventManager throughout input to
prevent UEventObservers from being added from the test process, which
doesn't have permissions.

Bug: 283782679
Test: atest InputManagerServiceTests
Change-Id: I023e5c1ef44367c2147978acae5f225c9f50f11c