Add DebugService.

This will dump the state of the telephony stack using:

  adb shell dumpsys activity service android.phone.TelephonyDebugService

The service is located in packages/app/Phone and TelephonyDebugService
instantiates DebugService and calls its dump method when asked
via the dumpsys command above.

Change-Id: I4d34c741544cafdadce2532de8b9c117a4d435a5