Fix an NPE in BatteryService

mCallback is possible to be null. Avoid registering null callbacks.

Bug: 175184519
Test: TH
Change-Id: Ic382d268f40a8ea31aaeedc7a85b6dc9b4e6c210
Merged-In: Ic382d268f40a8ea31aaeedc7a85b6dc9b4e6c210
(cherry picked from commit 780ca87adc239f295a26b7f0e89502a7f8758878)
1 file changed