Reference docs by ROOT_ID and DOC_ID; recents.

The same document may be present with different sematics under
multiple storage roots, so always reference using both ROOT_ID and
DOC_ID.  This enables backends to revoke permissions for an entire
root, such as when an account is removed.

Start building provider to remember recently accessed documents.

Change-Id: I75befa2e61393dec12fcc7fd27f631fcddae46fa
diff --git a/AndroidManifest.xml b/AndroidManifest.xml
index 453ef45..b88099e 100644
--- a/AndroidManifest.xml
+++ b/AndroidManifest.xml
@@ -21,6 +21,11 @@
             </intent-filter>
         </activity>
 
+        <provider
+            android:name=".RecentsProvider"
+            android:authorities="com.android.documentsui.recents"
+            android:exported="false" />
+
         <!-- TODO: remove when we have real clients -->
         <activity android:name=".TestActivity" android:enabled="false">
             <intent-filter>