Define MotionEventFlag as an AIDL enum

This allows us to loop through all enum values defined in AIDL in native
code in a unit test to verify that they are all redefined in various
languages. This will help catch missed flag redefinitions in presubmit
rather than as runtime crashes.

Bug: 346613247
Test: libinput_rust_test
Flag: NONE no behavior change
Change-Id: I48c5119e28a91315c3bbc561616eb48750bfb804