Add tests for BatteryMeterDrawable

Not meant to be an attempt at full coverage, just an example of
testing a Drawable.
To go farther with this, I would wrap the "logic" of the battery
appearance in a separate class for testing and this class would
reduce to the drawing operations, which are strongly coupled to
design and don't need much testing.

Test: runtest systemui
Change-Id: I5630e25face9a3ab5e30935fb3d7bab8f162d107