bt: support Sar backoff high resolution command in P22 projects

* set SarBackOffHighResolution = true for support power index
  from 0 to 80 for setting backoff in 0.25 dB step

Bug: 170369474
Test: sending sar command with power cap index between 20 ~ 80 and
confirm the SAR backoff is workable.

Change-Id: I56d75abcff55e93554bf656fff164b861fe0e3c2

bluetooth/bt_vendor_overlay.conf

@@ -2,3 +2,6 @@
 
 # Uart port name
 UartPort = /dev/ttySAC18
+
+# Sar backOff high resolution support
+SarBackOffHighResolution = true