Ensure pendingTransactions are applied if the VRI is torn down

When callers invoke the applyTransactionOnDraw, the code previously set
up an RT frame callback and only merged the transaction once a draw
started. However, if VRI is being torn down before a traversal starts,
the callback will never be invoked and the transaction will be left in
memory until GC. The cleanest way is to immediately apply the
transaction when the VRI is getting torn down to avoid leaving a stale
transaction with reference to SCs in it.

This required a bit of cleanup to ensure the UI thread still had access
to the SurfaceControl until a traversal was called. The new behavior
merges all incoming transactions from applyTransactionOnDraw into a
single Transaction. Then in the traversal path, it will either
piggy-back on the frame callback from sync transaction or create a new
one. The Transaction and flag are cleared in the traversal path since if
draw has been invoked, we are guaranteed the callback even if VRI dies.
This is because HardwareRenderer is not destroyed until everything
in the queue completes.

Test: presubmit
Bug: 248162583
Change-Id: I4d28b917545f530a068b237bf95de431afd749cb