Adding app op and intent action for external sources

Added an app op to control which package and uid can install apps on the
device and an intent action to launch the settings fragment.

Test: Will include in follow up CL, tracked in b/33792674

Bug: 31002700
Change-Id: Ic073495759d9867f8001a6c712e402398c53dfc9
diff --git a/core/res/AndroidManifest.xml b/core/res/AndroidManifest.xml
index 7f25cf3..8b4a576 100644
--- a/core/res/AndroidManifest.xml
+++ b/core/res/AndroidManifest.xml
@@ -2480,14 +2480,14 @@
         android:protectionLevel="signature" />
 
     <!-- Allows an application to request installing packages. Apps
-         targeting APIs greater than 22 must hold this permission in
+         targeting APIs greater than 25 must hold this permission in
          order to use {@link android.content.Intent#ACTION_INSTALL_PACKAGE}.
          <p>Protection level: normal
     -->
     <permission android:name="android.permission.REQUEST_INSTALL_PACKAGES"
         android:label="@string/permlab_requestInstallPackages"
         android:description="@string/permdesc_requestInstallPackages"
-        android:protectionLevel="normal" />
+        android:protectionLevel="signature|appop" />
 
     <!-- @SystemApi Allows an application to install packages.
     <p>Not for use by third-party applications. -->