Bugfix navigationbar size

[zhiweideng]

What’s the problem?

• FlashBar running in Android SDK 29, The problem is that the height of the NavigationBar is calculated incorrectly.
  

• FlashBar running in Android SDK 23, Setting the bottomMargin of FlashbarContainerView does not take effect.
  

Solution

• When FlashBar gravity == bottom, add FlashbarContainerView to the contentView of Activity, so that we don’t need to set the bottomMargin value of FlashbarContainerView, avoid calculating the wrong navigationBar height value and setting the bottomMargin of FlashbarContainerView unsuccessfully.