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

src/com/android/settings/security/InstallCertificateFromStorage.java

@@ -60,6 +60,7 @@ public class InstallCertificateFromStorage extends DashboardFragment {
 
     private static List<AbstractPreferenceController> buildPreferenceControllers(Context context,
             Lifecycle lifecycle) {
+        // TODO(eranm,victorhsieh): use "settings:controller" in xml and remove the following.
         final List<AbstractPreferenceController> controllers = new ArrayList<>();
         controllers.add(new InstallCaCertificatePreferenceController(context));
         controllers.add(new InstallUserCertificatePreferenceController(context));