Unified layer capture functions into one function

Cleaned up SurfaceComposerClient layer screenshot functions so there
is only function that takes a screenshot based on layer. It
accepts a LayerCaptureArgs and returns ScreenCaptureResults object.

Test: Layer screenshots in recents
Bug: 162367424

Change-Id: Iadee8024ebe895afb290a0ed798cbf8b637b30dd