Can switch to legacy GLES driver when ANGLE is system driver

Change the sense of the run-time switches so that ANGLE can be an
optional GLES driver or the default system GLES driver.  When ANGLE is
the system GLES driver, also handle the case of there being no legacy
GLES driver.

Test: logcat
Test: atest CtsAngleIntegrationHostTestCases
Test: atest CtsEffectTestCases:android.effect.cts.EffectTest#test5_effectCreate
Bug: 224558229
Change-Id: Ie53f5221d51b62db226c0c634537959612a06655