HFP: Reset battery level on HFP disconnect

* HFP is currently the solo source for battery level indicator
* Android loses track of ongoing battery level changes in remote device
  when HFP is disconnected
* Thus, remote device battery level information should be reset when
  HFP is disconnected before Android can obtain battery level information
  from other profiles
* Android should also send BATTERY_LEVEL_CHANGED intent with
  BATTERY_LEVEL_UNKNOWN when reset happens after Android have obtained
  battery level information from that remote device

Bug: 64027543
Test: Connect and disconnect HFP on Bluetooth device, unit test
Change-Id: I877a2f82311b55f09296bb39db166540ca9b973f
Merged-In: I877a2f82311b55f09296bb39db166540ca9b973f
(cherry picked from commit 02f24e80)