Add IS_GENERATED_GESTURE to MotionFlags in input.rs

Because IS_GENERATED_GESTURE is actually used in touchpad gestures, it
is a legitimate flag. We should add it to MotionFlags; otherwise,
MotionFlags::from_bits will return a None, which is not the result we
want.

Bug: 335391081

Test: Enable inbound event verification and use two finger swipe on
touchpad, device it should not crash.

Signed-off-by: Linnan Li <lilinnan@xiaomi.corp-partner.google.com>
(cherry picked from https://partner-android-review.googlesource.com/q/commit:fc358ec0877fd60e15645fe2683fa5ad5067a8eb)
Merged-In: I9c1a3f52508c106837bcabef0e10c71f69d1dadf
Change-Id: I9c1a3f52508c106837bcabef0e10c71f69d1dadf