Updated SkiaGLRenderEngine stretch logic

Removed explicit flag to disable stretching
surfaceflinger layers with the stretch shader in favor
of leveraging a adb setprop to determine the type of
stretch to apply.

Updated some variable names + types as response
to earlier CL feedback

Bug: 184297961
Test: manual
Change-Id: I4f1d7a7b4a33095d920b3149eb474cbe07c87cb0