Use sysprops to control the creation of protected context.

Previously we create protected context as long as the extension exsists,
however, a lot of devices that get upgraded to Q won't have such support even
if they have this extension, and thus we add a new configuration to allow OEMs
to configure their devices based on hardware capabilities. If the property is
configured as false, then we won't create protected context.

BUG: 35315015
Test: Verify with Youtube and ExoPlayer with protected contents.
Change-Id: I3cc11081ea9457c35f2380d4361be47a03438746
diff --git a/services/surfaceflinger/sysprop/SurfaceFlingerProperties.sysprop b/services/surfaceflinger/sysprop/SurfaceFlingerProperties.sysprop
index fe6dc93..830c03e 100644
--- a/services/surfaceflinger/sysprop/SurfaceFlingerProperties.sysprop
+++ b/services/surfaceflinger/sysprop/SurfaceFlingerProperties.sysprop
@@ -307,3 +307,11 @@
     access: Readonly
     prop_name: "ro.surface_flinger.use_smart_90_for_video"
 }
+
+prop {
+    api_name: "enable_protected_contents"
+    type: Boolean
+    scope: Internal
+    access: Readonly
+    prop_name: "ro.surface_flinger.protected_contents"
+}