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: I359b37daa96eb6f8424bde530bb1ac79affd1b04
diff --git a/private/property_contexts b/private/property_contexts
index 1ad19c1..ec6171f 100644
--- a/private/property_contexts
+++ b/private/property_contexts
@@ -1056,6 +1056,7 @@
 ro.hardware.consumerir           u:object_r:exported_default_prop:s0 exact string
 ro.hardware.context_hub          u:object_r:exported_default_prop:s0 exact string
 ro.hardware.egl                  u:object_r:exported_default_prop:s0 exact string
+ro.hardware.egl_legacy           u:object_r:graphics_config_prop:s0 exact string
 ro.hardware.fingerprint          u:object_r:exported_default_prop:s0 exact string
 ro.hardware.flp                  u:object_r:exported_default_prop:s0 exact string
 ro.hardware.gatekeeper           u:object_r:exported_default_prop:s0 exact string