GraphicsEnvironment: Refactor setupAngle for system ANGLE

This CL updates the code to make it clear which ANGLE choice has
priority, and allows system ANGLE to be overriden multiple ways:

1. settings for ANGLE
2. persist properties for EGL
3. readonly properties for EGL

The selection is more complicated than we'd like, but hopefully
clearer now, and more functional.

Written in conjunction with yuxinhu@google.com.

Flag: android.os.query_angle_choice_flag
Test: make, flash, boot, use ANGLE settings
Bug: b/408439360
Change-Id: I0b736886ebea57dc7e2ff3bc55e28de85ea32d3b