[BottomAppBar] Fix FAB location when RTL changes. (#6644)
[BottomAppBar] Fix FAB location when RTL changes. (#6644)

If the `semanticContentAttribute` for an MDCBottomAppBarView changes
during its lifetime, it won't correctly reposition the FAB. Instead of
checking the interface layout orientation once during initialization, it
should check every time it needs to perform layout.

|Before|After|
|---|---|
|<img width="380" alt="testfloatingbuttontrailingrtl_11_2-develop" src="https://user-images.githubusercontent.com/1753199/52929433-6812c800-3312-11e9-8505-07933e1ee077.png">|<img width="380" alt="testfloatingbuttontrailingrtl_11_2 2x" src="https://user-images.githubusercontent.com/1753199/52929438-6ba64f00-3312-11e9-805f-edc2e68cf1a5.png">|

    
Fixes #6642
material-components/material-components-ios,material-components/material-components-ios,material-components/material-components-ios,material-components/material-components-ios,material-components/material-components-ios