Prevent potential NPE when forcing stop package during boot

Some test cases may send package removal command before boot
complete, e.g. around SecondaryZygotePreload stage. Then
ActivityManagerService#setWindowManager is not called yet so
mRootWindowContainer in ATMS is null.

Bug: 219066637
Bug: 186665301
Test: Restart device by "adb shell am restart".
      And keep sending "adb uninstall $validPackageName".
      The device should still boot successfully without crash.
      And the target package is also removed successfully.
Change-Id: I2711fabb0755b53a9817ce81ab553e5ce2a8c389