Fixes null-ref when trying to start Guest session.

CarUserSwitchingDialog threw null-ref because Guest didn't have
a default avatar.

1. Adds null guard in case default avatar is not set.
2. Populates default Guest avatar correctly.

Fixes:77546931
Fixes:78588087
Test: Manual, observing the default icon when starting a guest session.
       Manual, observing the switching dialog icon without a default icon.

Change-Id: Iacbfcc65147fdbc5515c0d96eb613fb17543f6b3
diff --git a/packages/SystemUI/src/com/android/systemui/statusbar/car/UserGridRecyclerView.java b/packages/SystemUI/src/com/android/systemui/statusbar/car/UserGridRecyclerView.java
index 1148fad..5768fa2 100644
--- a/packages/SystemUI/src/com/android/systemui/statusbar/car/UserGridRecyclerView.java
+++ b/packages/SystemUI/src/com/android/systemui/statusbar/car/UserGridRecyclerView.java
@@ -207,7 +207,7 @@
 
                 // If the user selects Guest, start the guest session.
                 if (userRecord.mIsStartGuestSession) {
-                    mUserManagerHelper.switchToGuest(mGuestName);
+                    mUserManagerHelper.startNewGuestSession(mGuestName);
                     return;
                 }
 
@@ -241,8 +241,7 @@
 
         private Bitmap getUserRecordIcon(UserRecord userRecord) {
             if (userRecord.mIsStartGuestSession) {
-                return UserIcons.convertToBitmap(UserIcons.getDefaultUserIcon(
-                    mContext.getResources(), UserHandle.USER_NULL, false));
+                return mUserManagerHelper.getGuestDefaultIcon();
             }
 
             if (userRecord.mIsAddUser) {