* [ColorThemes] Use umbrella header instead of MDCColorScheme
* [Application] Use umbrella header instead of class header
* [Button] use umbrella header in private header
* [Typography] use umbrella header in private header
* icons using umbrella header in individual icon class
* [Icons] added header to umbrella header
* [collection] umbrella header instead of class header in test
* [Collections] use umbrella header in private header
* [ActivityIndicator] use umbrella header in private header
* [ButtonBar] use umbrella header in private header
* [Dialog] use umbrella header in private header
* [featurehighlight] use umbrella header in private header
* [snackbar] use umbrella header in private header
* [textFields] use umbrella header in private header
* [thumbtrack] use umbrella header in private header
* [Overlay] use umbrella header in private header
also removed spurious ’s’ at end of MaterialOverlay.h file
* [slider] use umbrella header in private header
* [buttonBar] use umbrella header in private header
* [slider] use umbrella header in test
* [Overlay] fixed overlay reference to use umbrella header
* [CollectionCells] Add accessibility hint in edit mode
When editing, selectable/deselectable cells now give an accessibility
hint: "Double tap to select.". This helps users to know what they can do
while in edit mode.
Related: https://github.com/material-components/material-components-ios/issues/1257
* Add tests