onPostComposition doesnt need to run on layers without queued frames

If we didn't latch a buffer with this frame, we will just return
early from onPostComposition and so we can safe traversal overhead
by just iterating mLayersWithQueuedFrames directly.

Test: Existing tests pass. simpleperf
Bug: 186200583
Change-Id: I8f72100f15d2b381e2480ee4c40f6ba79030e092