Update RenderEngine blur to snap an image of the full surface.

Snapping a subregion of a surface causes Skia's GPU surfaces to make
a copy of the pixels into a new buffer so that the dimensions of the
buffer match the requested size. By requesting a basic snapshot this
CL avoids any uncessary framebuffer copies.

Also refactor the code to start consolidating where blurs are used.

Test: tracing SysUI blurs
Bug: 176903027
Change-Id: I72a9e442e50bc6d945c601efca1fdd2ce7cdfcd1