Fixes touch ripples on media buttons.

This change makes touch ripples appear correctly on the action buttons
on media notifications.  This involves the following changes:

- NotificationViewWrapper.onReinflated() sets the notification content
root's background to transparent rather than null when transferring the
background color to the NotificationBackgroundView.  This gives the
ripples a surface to draw on.
- The RemoteViews for the media templates are changed to use a fixed
set of buttons instead of removing and re-adding them on every update.
This prevents the update caused by whatever action the tap invokes from
interrupting the ripples.
- A method is added to RemoteViews allowing the ripple color to be
specified.  This allows us to change the ripple color to match the
foreground color of the controls, which is necessary for the ripples to
show up against a dark background.

Test: manual
Change-Id: I907f9a1a75efb48da496133ad357fc5150de4410
Fixes: 35856702