Send BTPhoneService update when active/background calls switch.

This sends an update if the call-on-hold changes (when active and
background calls swap) as well as using a proper held-call count instead
of hardcoded 1 when there are multiple calls on hold.

This is needed to pass BT PTS test TC_AG_TWC_BV_03_I.

Bug: 17962996
Change-Id: Ia6d5f649020837a1ca37d0895f8844fcdec04446