bluetooth/native: Accept and report battery HF indicator value

HF indicator 2 (see [assigned-numbers], Hands-Free Profile) is able to
report battery percentage at 1% intervals (in range [0, 100]), contrary
to the `+XAPL` `+IPHONEACCEV` extension which only supports 10%
increments.  This does not guarantee increased granularity however, as
peers may still be limited to imprecise battery measurements internally
or round to coarser percentages.
Supporting both additionally broadens the range of devices for which PA
can report its battery level.


Part-of: <pulseaudio/pulseaudio!482>
2 jobs for battery-provider in 2 minutes and 22 seconds (queued for 7 seconds)