test_power: Add CURRENT properties

CURRENT is really general in other battery drivers,
Android also has an interface to monitor CURRENT, so let's
add it into test framework.

The default value (1.6A) is just a random but reasonable value.

Signed-off-by: Lecopzer Chen <lecopzer.chen@mediatek.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>