InputManagerService: Refactor native methods to be testable

We move the native methods of InputManagerService to a separate class
and interface, which allows it to be mocked for unit testing.

We also add an Injector class to allow for test depenedency injection.

Bug: 195528323
Test: build and flash
Change-Id: I9ae42a85addeac5ccf1aa1ed23956bdabd615531