InsetController: Release leashes from RenderThread

We handle changes to the leashes from the UI thread, but use
the same SurfaceControl wrapper object from the RenderThread with
SyncRtSurfaceTransactionApplier. This means that at the time
we release a SurfaceControl from the UI thread we might have already
scheduled a SyncRtSurfaceTransactionApplier to use it, and actually
that could be in the process of running, leading to racy access and
crashes. To fix this we release the SurfaceControl from the RenderThread
so that it happens behind all existing operations.

Bug: 151086678
Test: Existing tests pass.
Change-Id: I2308d1c64f3f368c32587f99ddfb9e05955f821f