Floss: Propagate HFP battery info into Floss

Forward battery info sent via AT+BIEV commands which covers devices not
using vendor-specific AT commands for HFP battery transmission.

Bug: 233101220
Tag: #floss
Test: manual and ./build --target test
Change-Id: Iee03eeab8c706ef817f5b6802dca539fa8de4cf8