[Feat] (MG_Util/SelfTest, android-plugin): driver POST self-test screen

Opening a MobileGL plugin APK now shows a POST screen that probes the
device's GLES and Vulkan drivers independently against MobileGL's
expectations - a device may satisfy only one backend - and reports a
per-backend verdict (OK / DEGRADED / UNSUPPORTED) with per-check rows.

The GLES probe builds its own ES3 pbuffer context on the system driver
and reuses FillInGLESCapabilities, including the indirect-draw
gl_InstanceID semantics probe; the Vulkan probe checks instance/device
requirements and the optional features each DirectVulkan path degrades
without. Results serialize as ASCII-safe JSON through a JNI entry in
libMobileGL.so; PostActivity renders them and caches the run per
process (single-flight, rotation-safe). PluginActivity keeps its
NoDisplay stub but the launcher entry moves to the POST screen; FCL
plugin discovery reads application meta-data and is unaffected.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

android-plugin/app/src/main/AndroidManifest.xml

@@ -7,15 +7,21 @@
         android:theme="@style/Theme.MobileGLPlugin">
 
         <activity
-            android:name=".PluginActivity"
+            android:name=".PostActivity"
+            android:configChanges="orientation|screenSize|screenLayout|smallestScreenSize|uiMode"
             android:exported="true"
-            android:theme="@style/Theme.MobileGLPlugin.NoDisplay">
+            android:theme="@style/Theme.MobileGLPlugin">
             <intent-filter>
                 <action android:name="android.intent.action.MAIN" />
                 <category android:name="android.intent.category.LAUNCHER" />
             </intent-filter>
         </activity>
 
+        <activity
+            android:name=".PluginActivity"
+            android:exported="true"
+            android:theme="@style/Theme.MobileGLPlugin.NoDisplay" />
+
         <meta-data
             android:name="fclPlugin"
             android:value="true" />