Add debug property to safe guard ANGLE developer option UI.

Add a debug property to guard ANGLE developer option UI, only when the
debug property is enabled will ANGLE developer option UI be enabled.

Add further clarification in the text to indicate that this is an
experimental feature.

Bug: b/287909344
Test: atest -c GraphicsDriverEnableAngleAsSystemDriverControllerJUnitTest
Test: atest -c GraphicsDriverEnableAngleAsSystemDriverControllerTest
Change-Id: Ife31f4b0426f3ce107f86cfbaf3f1aeca567e250

src/com/android/settings/development/graphicsdriver/GraphicsDriverSystemPropertiesWrapper.java

@@ -41,4 +41,13 @@ interface GraphicsDriverSystemPropertiesWrapper {
      * SELinux. libc will log the underlying reason.
      */
     void set(@NonNull String key, @Nullable String val);
+
+    /**
+     * Get the boolean value for the given {@code key}.
+     *
+     * @param key the key to lookup
+     * @param def the default value in case the property is not set or empty
+     * @return if the {@code key} isn't found, return {@code def}.
+     */
+    boolean getBoolean(@NonNull String key, @NonNull boolean def);
 }