Remove SF statelock and legacy layer uses in captureLayers

Minimizes the areas that we accesss mStateLock on the screenshot
path by grouping all work that requires mStateLock in the same
stage of the screenshot. Migrates away from legacy layer use and
corrects floating point precision truncation with storing frame
scaling factor in screenshot arg's size.

Bug: b/377758217
Test: atest SurfaceFlinger_test
Flag: EXEMPT, refactor, legacy code removal
Change-Id: Ib2d1f9e47043096fa7cb83ca4eedb5e32d68f54e