power: vm-bms: Add get function for CURRENT_NOW property

CURRENT_NOW property gives amount of current flowing through
the battery.
Add a function to read CURRENT_NOW power_supply property.

CRs-Fixed: 658789
Change-Id: I2bfe02d60e2c2649b64b7cc206ef5751f0922db1
Signed-off-by: Ashay Jaiswal <ashayj@codeaurora.org>