Add "ro.hardware.egl_legacy" for ANGLE system driver

This supports the ability to switch between ANGLE and a legacy GLES
driver in cases when transitioning from a legacy GLES driver to ANGLE
as the system driver.  With ANGLE as the GLES system driver, the
platform needs a way to identify the legacy GLES driver, so that it
can be used for particular applications.

Test: CtsAngleDeveloperOptionHostTest
Bug: 224558229
Change-Id: Ifad93145cb3fc7d14b75472484b008d00656edc1