Do not drop stale events for gestures in progress

Before this CL, stale events would get dropped by the dispatcher and
cause inconsistent event streams to be sent further down the pipeline.
This would cause a crash.

In this CL, we prevent dropping stale events when the gesture is
currently in progress. This ensures that only events that start a new
gesture could be dropped due to being stale.

Test: TEST=inputflinger_tests; m $TEST && $ANDROID_HOST_OUT/nativetest64/$TEST/$TEST
Bug: 308153460
Change-Id: I8348dc9e175214dab3f68f33ad52a117637c2f9a