Prevent mPendingTargetUserIds queue breaking when a switch fails.

Prevent `mPendingTargetUserIds` queue breaking when a switch in
the queue fails.

This command chain will leave user 10 in foreground. And user 11
in `mPendingTargetUserIds`. Because the 3rd command fails and
re-iteration breaks.

```
adb shell am switch-user -w 0
adb shell am switch-user 10
adb shell am switch-user 10
adb shell am switch-user 11
```

Bug: 411340343

Test: atest UserControllerTest

Test: adb shell am switch-user -w 0
      adb shell am switch-user 10
      adb shell am switch-user 10
      adb shell am switch-user 11

Test: adb shell am switch-user -w 0
      adb shell am switch-user 10
      adb shell am switch-user 11
      adb shell am switch-user 11
      adb shell am switch-user 10

Flag: EXEMPT bugfix

Change-Id: I6a562af0c9b975b9e328cd3308885df4b6b17796