power_supply: add batt_age_level property

Add batt_age_level property to support loading multiple profiles
based on the battery age level specified by the user.

Change-Id: I41e61c615f2d1df988a0de83e4e2dbddd7a36441
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>