Add classification to MotionEvent

The classification will be used to categorize the
current motion event stream. The classification may change from ambiguous to deep press,
for example.

Test: integration tested by printing to logcat whenever CLASSIFICATION_DEEP_PRESS
is present.
Test: atest libinput_tests inputflinger_tests
Bug: 62940136

Change-Id: I2d93583bab42162e1fd1e95ebf624ef3ab9a58ec