Allow drivers to alter the default driver name on rsdHalInit

Bug: 20894432

Permit partner drivers via Context::setDriverName to choose their
own mDriverName rather than the default OVERRIDE_RS_DRIVER string.

This can only be done once from within rsdHalInit otherwise it will
be ignored and the default behaviour will apply.

Change-Id: I49b81e60f7983a81f0e71fbef4b46f9bfc834849
diff --git a/rsDriverLoader.cpp b/rsDriverLoader.cpp
index 5bfb8e5..7b728b6 100644
--- a/rsDriverLoader.cpp
+++ b/rsDriverLoader.cpp
@@ -207,7 +207,7 @@
     }
 
     // Only map in the actual driver name if we successfully load the runtime.
-    mDriverName = filename;
+    setDriverName(filename);
 
     return true;