InputDispatcher: Remove offsets for gesture monitors

Offsets for gesture monitors are not used and are always initialized to
their default values of 0. We remove these offsets, allowing us to
remove the TouchedMonitor class as well.

Bug: 179274888
Test: atest inputflinger_tests
Change-Id: I1eea9a856fc5c70f18f50eda01d256cda8d17c2d