Added functionality to select type of certificate to be installed from the Settings app

This is part of the changes to improve the UX and language for installing certificates.
Previously, the different types of certificate used the same installation flow. This CL
introduces a new settings page, where the type of certificate to be installed can be selected.

Bug: 139173976
Test: Atest com.android.settings.security
      manual testing from Settings by selecting the certificate type
	preference and ensuring the installation flow still worked as expected.

Change-Id: Iea7c91aa3801e429f0e22d29469958f4151b3cba

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

@@ -0,0 +1,35 @@
+/*
+ * Copyright (C) 2017 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.android.settings.security;
+
+import android.content.Context;
+import android.os.UserManager;
+
+public class InstallCertificatePreferenceController extends
+        RestrictedEncryptionPreferenceController {
+
+    private static final String KEY_INSTALL_CERTIFICATE = "install_certificate";
+
+    public InstallCertificatePreferenceController(Context context) {
+        super(context, UserManager.DISALLOW_CONFIG_CREDENTIALS);
+    }
+
+    @Override
+    public String getPreferenceKey() {
+        return KEY_INSTALL_CERTIFICATE;
+    }
+}