Use Theme consistent styles in FoldableLinearLayout

For support of the dark and light themes.

Also:

Redefine mFolded and call it mIsFolded.  Previously,
the view started with mFolded = false (which implies to me
the initial state is unfolded) and yet the view
started in a folded state, which seemed contradictory.

Create updateFoldedState() with code from onClick() (In
preparation for subsequent commit.)