Disable sensors when an app goes to background v2

This change is a revision of a previous version of the
implementation in ag/10737391. In this version, we
perform the book-keeping of backgrounded apps in the
SensorDevice.cpp (rather than SensorEventConnection.cpp),
to avoid unintended problems with the locking assumptions
in the Sensor Service code.

Bug: 74395023
Test: 1) Subscribe to sensor on sensorlogger
      2) Put app to background and wait
      3) Verify subscription is disabled once app is idle
      4) Put app to foreground and verify app can receive sensor samples
Test: Verify above test while a_sns_test streaming samples
Test: Verify above test with sensor service restrict and privacy mode
Change-Id: Ic44058e606868a665d5aa88a75211d1eba8d4950