Grant external storage permissions to SystemUI.

Used for taking screenshots and playing notification ringtones.

Bug: 6381589
Change-Id: Ib1a5ad17edbeab984bbab25168d81eb99deba952

Author: jsharkey

packages/SystemUI/AndroidManifest.xml

@@ -3,6 +3,8 @@
         coreApp="true">
 
     <uses-permission android:name="android.permission.RECEIVE_BOOT_COMPLETED" />
+    <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />
+    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
 
     <uses-permission android:name="android.permission.INJECT_EVENTS" />
     <uses-permission android:name="android.permission.WRITE_SETTINGS" />