Create MailPreferenceActivity

Enable test for GeneralPrefsFragment
Add test instance support to MailPrefs
Fix some values that weren't getting .apply()'d

b/9566150

Change-Id: Ia636c748c67518b0a6e3d436c8bfd09620fda6fc
diff --git a/AndroidManifest.xml b/AndroidManifest.xml
index a7b39e9..f2b2976 100644
--- a/AndroidManifest.xml
+++ b/AndroidManifest.xml
@@ -122,6 +122,11 @@
             </intent-filter>
         </activity>
 
+        <activity android:name=".ui.settings.MailPreferenceActivity"
+                  android:label="@string/activity_preferences"
+                  android:exported="false">
+        </activity>
+
         <provider
                 android:authorities="com.android.mail.accountcache"
                 android:label="@string/account_cache_provider"