Refactor for moved batterystats AIDL.

Moving this out of libbinder saves 4KB in 44 processes (on cuttlefish) =
176KB of private dirty memory (that is, this takes libbinder from 84 to
80 KB of private dirty memory). This also adds the cost of a new library
in two processes.

Bug: 183654927
Test: boot
Change-Id: I37644388ac7a7807198f5fcfae4186ec418eb862