Add InputReporter interface to InputDispatcher

This adds a new library to inputflinger, libinputreporter, that handles
events reported from InputDispatcher.

The CL makes InputDispatcher notify InputReporter for two events:
- Unhandled Keys: Keys that were not handled by the system or apps. A key
event is unhandled if:
   - The event was not handled and there is no fallback key; or
   - The event was not handled and it has a fallback key,
       but the fallback key was not handled.
- Dropped Keys: Key events can be dropped for several reasons, which
can be seen in InputReporter::DropReason.

Currently, the default implementation of InputReporter does nothing when
it recieves a dropped or unhandled key notification. The InputReporter
is created via its factory method, createInputReporter().

Bug: 119312287
Test: manual: Build, and verify using log output
Change-Id: I90ac525d648bb75e0bc8532f68f6a9e4ab51d54b