power_supply: Move battery_type under string properties

Currently, battery_type is treated as an integer property as it
is not in the group of string properties. Move it to the right
place after model_name from where string properties start.

Change-Id: Ib97663ed9b3b5c3f6b3ef4fe49a1e2d1d536ebc6
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>