V4L2DecodeComponent: abandon pending work when drain is done.

Originally we forced the component into error state if there are any
pending frames when drain is done. That means these frames are either
no-show frames or dropped. It doesn't affect the following decoding.
This CL changes to abandon pending frames instead of reporting error.

Bug: 184211876
Test: android.media.cts.AdaptivePlaybackTest

Change-Id: I18511f14f70579718f434f316701ee1b77c1f6f0