HFP : Correct headset battery level percentage calculation for vendor specific XEVENT event

* XEVENT report the battery level is a zero-based integer that designates the rough charge level
  think of this as the number of bars on the hypothetical battery meter in the headset.
* XEVENT report the numberOfLevels is an integer that describes the number of battery levels possible.
  If the hypothetical battery meter has a range of 0-4, this value will be 5.

Bug: 109652276
Test: paired with Plantronics headset and check the headset battery
percentage.

Change-Id: Ifdd8560626ecd124360973e0ef8fd2b5a2d6edde