Revert "Adjust media buffer GC threshold"

This reverts commit c17496bbe1a8fcb56b36708d27d5c0173bb69c19.

Reason for revert: Cobalt can remove some samples (not yet written to decoder) with lower media buffer GC threshold.

Change-Id: Ida3e78eb97312712f8940f158dc302a59bd33c49