Refactor of screenshot code on main thread.

Create helper functions to improve readability of what is scheduled on
the SurfaceFlinger main thread. This will allow for cleaner changes in
reducing the calls on the main thread for screenshots. Changes include
some renaming for better clarity.

Bug: b/294936197
Test: presubmit
Test: atest SurfaceFlinger_test
Change-Id: I729b62b3fc1919b5b048b7992b4aead336e87d7f