Simplify pointer handling code in InputDispatcher

Some of the code in dispatcher can be simplified.

Some fixes here:
1. Use 'removeTouchingPointer' instead of implementing the same logic
   from scratch.
2. Remove `addTouchingPointer` API since there's a similar API called
   'addTouchingPointers'.
3. Use MotionEvent::getActionIndex instead of a custom function that
   does the same thing.

Bug: 211379801
Test: TEST=inputflinger_tests; m $TEST && $ANDROID_HOST_OUT/nativetest64/$TEST/$TEST
Change-Id: I1fec64be6e9da11199c55a96cac17ee5e6c14b82