Add multi-device tests to dispatcher

In this CL, several multi-device tests are added to InputDispatcher.
This is done for the following reasons:

1. Increase test coverage of dispatcher
2. Reduce the diff in the upcoming "multi-device input" CL
3. Show the difference in behaviour from the old code to the new one

This should simplify the review of future CLs because the difference in
behaviour will be easy to spot.

Bug: 211379801
Test: TEST=inputflinger_tests; m $TEST && $ANDROID_HOST_OUT/nativetest64/$TEST/$TEST
Change-Id: Ifadfc35eee43297305387e2cb79bc671259fd0bf