Fix cannot play issue after seeking for Vorbis Playback.

Root cause:
1. Play a Vorbis file, and seek several times
2. Resubmit csd-0, mState is cleared and mInputBufferCount is
   set to 1 in SoftVorbis
3. Flush again, mInputBufferCount will not reset to 0 due to
   mState is null
4. Resubmit csd-0 and csd-1, csd-0 will be used to do
   _vorbis_unpack_books due to mInputBufferCount is 1, and
   error occurs

Solution: Reset mInputBufferCount in onPortFlushCompleted

Bug: 133472577
Test: Play a Vorbis file, seek several times, and check if the
device can play it normally
Change-Id: I4feff27ffb8a69b17b8009ca91350339691c5100