Ref: wordpress-mobile/gutenberg-mobile#935

This PR focuses on the cells of the bottom sheets, providing better labeling, hints and action handling.

• The Cells with no value has their label as default accessibility label.
• For Cells with value, is needed to provide a properly localized accessibility label.
• For SwitchCells, the proper localized accessibility label is needed too, indicating the switch state.

For both the Cells with value and SwtchCells, the manual handling of the labels is not optimal, but required if we want to properly localize the whole sentence.

I believe that for these cases we can make an exception, and generically concatenate label and value as the accessibility label for cells with value.

Same for switches, we can generically concatenate the localized switch state with _x(), indicating the context of the On, Off strings.

This will save us from possible mistakes in the future, and will help with an easier accessibility maintenance.

I left this PR implementing the first case to show the point. But I'd happily edit it to automatically handle accessibility labels in BottomSheet cells. See the changes on image/edit.native.js and link/modal.native.js.

EDIT: Made changes to handle accessibility labels internally.

To test:

• With VoiceOver ON, select a paragraph block.
• Activate the link button in the toolbar.
• Check that the labels are read correctly, with Empty when correspond.
• Check that the cell with switch is toggled with double tap.
• Select an Image block with an image.
  • Recommended to deactivate VoiceOver to exit the BottomSheet :(
• Activate the Image Settings button from the inline tool bar.
• Check that the labels are read correctly, with Empty when correspond.