Battery meter: remove bolt color compromise & improve pct.

 - Bolt remains opaque (white) below a threshold level, otherwise
   transparent.
 - Draw the entire shape using a path, removing the need for a
   software layer.
 - Use a similar approach to make the percentage text readable,
   but this still requires a software layer (since text is involved).

Bug: 12131168
Change-Id: Ifde5e99121155bf1be171f44b2c80c116b17c9e7