Explicitly export content providers

In a future version of Android, the default value for
<provider android:exported> will change from true to false.
Explicitly set android:exported="true"

Bug: 3306452
Change-Id: Id23f16a8527ba97be1d8617ccfc4346734f695d0
diff --git a/AndroidManifest.xml b/AndroidManifest.xml
index 05df6cd..7a1ce39 100644
--- a/AndroidManifest.xml
+++ b/AndroidManifest.xml
@@ -58,7 +58,7 @@
     <application android:process="android.process.media"
                  android:label="@string/app_label">
         <provider android:name=".DownloadProvider"
-                  android:authorities="downloads">
+                  android:authorities="downloads" android:exported="true">
           <!-- Anyone can access /my_downloads, the provider internally restricts access by UID for
                these URIs -->
           <path-permission android:pathPrefix="/my_downloads"