Introduce monitoring behavior changes for USI devices (2/n)

- USI devices are always expected to report a UEvent whenever the
  battery state changes. This signals that the current stylus battery
  level has changed. This means we should only consider the USI battery
  state as valid when we receive a new UEvent.
- Once a stylus battery has been reported, the sysfs battery node will
  report the last value perpetually, until the next change, even if the
  stylus is no longer being used. To handle this, we set a validity
  period, where the USI battery is only valid for a fixed time period
  (one hour) after the last time the battery changed.

DD: go/inputdevice-battery-notifications

Bug: 243005009
Test: atest FrameworkServicesTests
Test: manual with USI 2.0 device
Change-Id: I1c43a04208601c3dda19aa18f1ede3303ececc97