Check for null application context in UserManager.

UserManager currently always uses getApplicationContext for its
internal context object, but that method can return null under certain
circumstances. This CL adds a null check, so that the original context
is used if the app context is null.

Bug: 232179155
Bug: 233915566
Test: atest UserManagerTest
Test: install Apex Launcher and App Hider on device with work profile
Change-Id: I26bcf693da8f5bc53126f79f9e6ffb17bd87fc84

core/java/android/os/UserManager.java

@@ -1971,7 +1971,8 @@ public class UserManager {
     /** @hide */
     public UserManager(Context context, IUserManager service) {
         mService = service;
-        mContext = context.getApplicationContext();
+        Context appContext = context.getApplicationContext();
+        mContext = (appContext == null ? context : appContext);
         mUserId = context.getUserId();
     }