SurfaceComposerClient BLAST Transactions: Correctly merge mContainsBuffer

If we merge a transaction which does not contain a buffer in to a transaction
which already contained a buffer, the resulting transaction still needs to
have mContainsBuffer=true, otherwise we won't properly cache the buffers.

Bug: 153561718
Test: Existing tests pass. Manual test of BLASTSyncEngine.
Change-Id: Ic63aba390757285730cb19406dc5c6b73d0c3437