Move stale event check into dispatcher policy

This cleans up the dispatcher interface a bit, but also, it allows us to
change the timeout more easily for tests.

Test: TEST=inputflinger_tests; m $TEST && $ANDROID_HOST_OUT/nativetest64/$TEST/$TEST
Bug: 308153460
Change-Id: Ieb4c7a9d53b62b0e0a73c4f43038db949a821993