Allow specifying ANGLE in manifest via meta-data

This CL allows the application's manifest to specify whether
to use ANGLE or native drivers for GLES via meta-data.

To enable, place the following within <application> and it
will be detected by the platform:

    <meta-data
        android:name:"com.android.angle.GLES_MODE"
        android:value:"angle" />

The manifest can also specify "native" to opt-out of
ANGLE, but native will be the default for Q release.

This method is in service of the tech preview of
ANGLE for Android. For Q release, we should move this
to the Android API.

Test: Manual, verified manifest can specify angle/native/foo
Change-Id: Iedc081a3700e05b8c391016704d0c9d623a4e76d
(cherry picked from commit e1cff35269c19020de5bf15028f0c4051ee508c5)
diff --git a/opengl/libs/EGL/Loader.cpp b/opengl/libs/EGL/Loader.cpp
index f5e0ad9..90721b8 100644
--- a/opengl/libs/EGL/Loader.cpp
+++ b/opengl/libs/EGL/Loader.cpp
@@ -486,12 +486,15 @@
 
     android_namespace_t* ns = android_getAngleNamespace();
     const char* app_name = android_getAngleAppName();
+    const char* app_pref = android_getAngleAppPref();
     bool developer_opt_in = android_getAngleDeveloperOptIn();
 
     if (ns) {
         // If we got a namespce for ANGLE, check any other conditions
         // before loading from it.
-        if (developer_opt_in) {
+        // TODO: Call opt-in logic rather than use pref directly
+        //       app_pref will be "angle", "native", or "dontcare"
+        if ((developer_opt_in || !strcmp(app_pref, "angle"))) {
             so = load_angle_from_namespace(kind, ns);
         }
     }