Reduce SF stateLock on screenshot path

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. Simplifies the captureDisplay functions
and RegionSamplingThread to only access the display on the main
thread with the state lock held. This removes the need to pass
around the display as a wp, which was originally necessary due to
sp<DisplayDevice> lifetime limitations.

Bug: b/377758217, b/159112860
Test: atest SurfaceFlinger_test
Test: atest CompositionTest
Test: atest RegionSamplingTest (libgui and sf)
Flag: EXEMPT, refactor
Change-Id: Ia88cf5dfbea35cd0531e469e4d01276ebce9feb4