Support installation of the new app source certificate

The new certificate can be installed from Settings ("Install a
certificate > App Source certificate").  The installation flow includes
a warning with user authorization to proceed, then a prompt for reboot
(now or later).

Installed certificate can be managed in "User credentials".  The name is
currently a hash of hex numbers.

Upon deletion, there will also be a promot for reboot (now or later).

Test: Only see the new setting entry if feature is enabled
Test: Install from Settings, see the expected file name in
      /data/misc/keysetore/user_0.  Reboot also works.
Test: Able to see the certificate in Settings after installed
Test: Able to delete the certificate, which triggers confirmation dialog
      to reboot.  Reboot works.
Test: add certificate, see dialog, "not now" / tapping elsewhere does
      nothing
Test: atest RestrictedEncryptionPreferenceControllerTest
Bug: 112038744

Change-Id: I7a4494ea0f243730df2212076588074d8774ae23

res/xml/install_certificate_from_storage.xml

@@ -62,6 +62,18 @@
 
         </Preference>
 
+        <Preference
+            android:key="install_app_src_certificate"
+            android:title="@string/app_src_certificate"
+            settings:controller="com.android.settings.security.InstallAppSourceCertificatePreferenceController">
+
+            <intent
+                android:targetPackage="com.android.settings"
+                android:targetClass="com.android.settings.security.InstallAppSourceCertificateWarning">
+            </intent>
+
+        </Preference>
+
     </PreferenceCategory>
 
 </PreferenceScreen>