Use proper string formatting for the "Active device" string

* The hard-coded "active" / "active(media)" / "active(phone)" strings
  are removed and replaced with proper XML-based string formatting.
* Added the appropriate strings for Bluetooth Audio Active Device status:
  "bluetooth_audio_active_device_summaries". For now those strings are
  marked as translatable="false" until the actual UI is finalized.
* Updated all "bluetooth_connected*" strings to include the new
  "active_device" component.
* Added unit tests for the new "active" strings.
  Also, updated existing unit tests to check the getConnectionSummary()
  strings by comparing against strings embedded within the unit tests.

Bug: 72317067
Test: Unit tests added: make RunSettingsLibRoboTests -j40
      Manual: two headsets and switching the active device
Change-Id: Ide639b5dfb45c1db8114155240f193249aeaf3be