healthd: Improve readability of BatteryMonitor::init()

Declare and assign in a single statement instead of in two separate
statements. This patch does not change any functionality.

Test: Compile-tested only.
Change-Id: I0798cc940f5f2ca329548568ab68e96891521dd8
Signed-off-by: Bart Van Assche <bvanassche@google.com>