Extract InputSettingsObserver from InputManagerService

The code to observe settings changes and pass them down the stack is
quite bulky and repetitive. Extracting it to a separate class makes
InputManagerService a little less unwieldy, and we can refactor it a bit
to use a map of observers to make the code less verbose.

Bug: 251196347
Test: atest inputflinger_tests
Test: change settings values and check the changes are applied
Change-Id: Ic94b311aed7281aedd86fc6dffbe56104cf90297