Drop sync buffers for subsequent sync requests

When making subsequent draw requests for the same window, drop
any existing buffers since they will be replaced by the new one.

Sometimes the "old" buffer will still be "in-flight" (either
drawing still or in binder). So, also drop incoming buffers
for seqIds that are earlier than the latest prepareSync.

Additionally, immediately apply any applyWithNextDraw that were
requested before the sync, but arrive after the sync started --
otherwise they can get put on pending (which waits for sync
apply) and thus also block the buffer queue.

Bug: 233625646
Test: run tests and check for ANRs
Change-Id: I156a9a73eea8346dc241b5d782a863b99127ca9f