Allow native driver to be ANGLE

There are ANGLE specific behaviors that need to be followed whenever
the driver in use is ANGLE. This change removes those behaviors from
the ANGLE load path and allows them to be applied to the "native"
driver if it is ANGLE.

This change removes the ability to select different ANGLE back-ends.
E.g. debug property: "debug.angle.backend" no longer works.
Only Vulkan will be used. The other back-ends are not used or tested
on Android and would be unusable on a production device where ANGLE
is the one and only OpenGL driver.

Test: atest CtsAngleIntegrationHostTestCases
Bug: b/154237217
Change-Id: Ia792f63d8c7b9411056b9277ff4b37fa566335f5