Make ANGLE loadable from system image.

Previously ANGLE was built as part of vendor partition, and hence when
it was the default OpenGL ES driver, the loader should use sphal
namespace to dlopen it. However, having ANGLE in vendor partition also
means Android OS relies on updates of the vendor partition to get ANGLE.
While in general ANGLE has an implicit dependency on the Vulkan driver
in the vendor partition, we should be able to update it outside of the
vendor parition. This patch changes the loading the ANGLE binaries with
the assumption that ANGLE binaries is part of the system image. This
will give Android OS the benefit to ship and update ANGLE independently
from vendor partition and hence shipping ANGLE is no longer blocked by
updating the vendor partition. This will also allow Android to have
minimal requirements on ANGLE.

Bug: b/293503000
Test: atest CtsAngleIntegrationHostTestCases -c
Change-Id: If32dd9ff8b023dc975930cb7bce7b003c10618e4