Do pixel-perfect mapping on x86_64 devices only

This CL changes the matcher used for unit screenshot tests so that we do
pixel-perfect matching only on x86_64 devices, and use the MSSIM matcher
otheriwse. See comment in the code and b/237511747 for more information.

Bug: 237511747
Test: atest SystemUIGoogleScreenshotTests
Change-Id: Ia8915de33521035d46f1854580db830b11f94e5c