UiccSlot: Validate hotswap restart prompt component properly

Resources.getString(int) cannot return null, therefore instead let's
check if ComponentName.unflattenFromString() is null.

Test: Remove SIM card on a device with config_hotswapCapable set to
      false and config_iccHotswapPromptForRestartDialogComponent set to
      @null, notice that dialog prompting user to restart is shown.
Bug: 227459873
Change-Id: Id7c8c1bfad6bb25fe5e8f8b4bb7be91e2484dd17