Consolidate layer snapshot functions for screenshots

Merges layer snapshot functions into 1 and directly calls into
it to get snapshots for screenshots, reducing the need to
pass around a snapshot function.

Bug: b/377758217
Test: atest SurfaceFlinger_test, atest CompositionTest
Flag: EXEMPT, refactor
Change-Id: I02236342f77102b500d11b02882023eb71cab9df