Also allow avoiding gfx acceleration at runtime

This change adds a way to make ActivityManager#isHighEndGfx() return
false at runtime by inspecting the value of ro.config.avoid_gfx_accel.
This is useful for running Android on Chrome OS devices, since a single
image can be made to run in both physical hardware and an emulator.

Bug: 65602168
Test: Added `ro.config.avoid_gfx_accel`, isHighEndGfx() returned false.
Change-Id: I1599ad9b385f9ef43f923886e6d5358ae07632e0