Flush events before stopping filter.

All the filter events in scheduler should be cleaned before stopping.
Sending events after stopping may cause problems if frameworks has
release some references.

Bug: 205763272
Test: atest VtsHalTvTunerTargetTest
Test: atest android.media.tv.tuner.cts
Change-Id: I2d3875b348035e14314034ff90aa8c066108affe