Fix updateBluetoothStateLocked comments

There was confusion as to whether updateBluetoothStateLocked
took an ActivityInfo representing a cumulative total or a delta.
It is confirmed that it is now a cumulative total. (It had been
a delta in the past, but no longer.)

Bug: 174818545
Test: N/A (just comments)
Change-Id: I62130f22250eb496164f94c0668d1ff65fed7710