Adding an api for apps to check whether they can install apps

Some apps may want to check whether they are trusted to install apps on
the device, so they can prompt the user to go to settings and mark them
as trusted before they do an intensive operation like downloading an
apk.

Test: cts-tradefed run cts -m CtsExternalSourcesTestCases

Bug: 31002700
Change-Id: Icd9d04daa157e6733decba245ec251ce4acd4122
diff --git a/core/res/AndroidManifest.xml b/core/res/AndroidManifest.xml
index 6d48862..8f8acc1 100644
--- a/core/res/AndroidManifest.xml
+++ b/core/res/AndroidManifest.xml
@@ -2482,7 +2482,7 @@
     <!-- Allows an application to request installing packages. Apps
          targeting APIs greater than 25 must hold this permission in
          order to use {@link android.content.Intent#ACTION_INSTALL_PACKAGE}.
-         <p>Protection level: normal
+         <p>Protection level: signature
     -->
     <permission android:name="android.permission.REQUEST_INSTALL_PACKAGES"
         android:label="@string/permlab_requestInstallPackages"