Dispatch texture destruction off of render thread for screenshots

This is only enabled when the skia threaded variant renderengine is
enabled. This essentially defers texture deletion until after
drawLayers() is invoked, which shaves off time from the main SF thread.

Bug: 179959621
Bug: 180767535
Test: perfetto trace
Change-Id: I333091d8792fdd0dc01076c73af2956da9ee73df