Delete RenderProxy off of the cleaner thread

Speculation for the cause of an ANR. HardwareRenderer's
cleaner can block on RenderThread, which in turn blocks ART's
cleaner thread. This can potentially cause other blockages, but
it's also poor behavior to have such a long-running Cleaner anyway.

Avoid this by putting cleanup to CommonPool.

Bug: 191514384
Test: make, CtsUiRenderingTestCases still passes

Change-Id: I8190f7862528c3ac39ce636f6fca229322480968