Add global settings variables for ANGLE debug.

Introduce angle.egl.features to assist ANGLE debugging. The input
contains lists for each opt-in apps. The format is each list of features
is separated by a comma, each feature in each list is separated by a
colon, e.g.  feature1:feature2:feature3,feature1:feature3:feature5

Bug: b/169156341
Test: use adb shell command to verify
Test: atest android.gputools.cts.CtsRootlessGpuDebugHostTest
Test: atest CtsAngleIntegrationHostTestCases
Change-Id: I752047df1c2567b4ebf900c3acb4a965e35dd6c2