InsetSourceConsumer: Ensure we release Leash when finished.

SurfaceControl resources may be heavy weight, and so we explicitly
release them when finished. Specifically, we need to fix this to
keep the CloseGuard from complaining in strict-mode.

Bug: 150805473
Test: Existing tests pass
Change-Id: Ie3c609f79a953374a4d336465676cdc312679f93