Added a placeholder home activity for system user

Standard launcher can be uninstalled for system user. A low priority
placeholder home activity is used to make sure the system can always
boot. By default the component is disabled for all users, and is only enabled
for the system user at boot time.

Bug: 25726710
Change-Id: I97eed02c9e2bbd69a737cc1d203110bed837b803
diff --git a/core/res/AndroidManifest.xml b/core/res/AndroidManifest.xml
index 6bdf71b..5abcbab 100644
--- a/core/res/AndroidManifest.xml
+++ b/core/res/AndroidManifest.xml
@@ -2835,6 +2835,17 @@
                 android:process=":ui">
         </activity>
 
+        <activity android:name="com.android.internal.app.SystemUserHomeActivity"
+                  android:enabled="false"
+                  android:process=":ui"
+                  android:systemUserOnly="true"
+                  android:theme="@style/Theme.Translucent.NoTitleBar">
+            <intent-filter android:priority="-100">
+                <action android:name="android.intent.action.MAIN" />
+                <category android:name="android.intent.category.HOME" />
+            </intent-filter>
+        </activity>
+
         <receiver android:name="com.android.server.BootReceiver"
                 android:systemUserOnly="true">
             <intent-filter android:priority="1000">