Combine StylusCallback and StylusBatteryCallback.

These were initially separate to allow listening to
battery events separately from InputDevice events. However,
in practice, these events are always listened to together in
StylusManager. Separating the two does not provide any performance
improvement, and has in fact caused bugs (ag/21382305).

Bug: 269088577
Test: StylusManagerTest, StylusUsiPowerStartableTest,
StylusBluetoothPowerStartableTest

Change-Id: I2c83d8e7db93601ec9d32fc6b868396d073910f8