OpenGL: fix initializeAnglePlatform with built-in ANGLE driver

When ANGLE namespace doesn't exist and initializeAnglePlatform is
called, ANGLE driver is loaded as built-in gl driver in sphal namespace.
This change fixes the fallback path here.

Bug: 154237217
Test: atest CtsAngleIntegrationHostTestCases
Change-Id: I0e4ecf65fac0d91b5542d475439e1e6f9541b629
Merged-In: I0e4ecf65fac0d91b5542d475439e1e6f9541b629