Use SOURCE_CLASS_POINTER as the default source for MotionEvent

This CL also adds a test to ensure that a pointer source is used by
default when obtaining a MotionEvent, and that offsets are only applied
to pointer sources.

Bug: 207251886
Test: atest MotionEventTests
Change-Id: Id40accc6d1238c940f139a18b26fa277dd6e11f9