Dump contexthub in protobuf format

Add protobuf dumpsys of connmetrics. Primarily used by incident service
to capture an incident report proto.

Command to invoke (any of the following after lunch and env setup):
$ adb shell dumpsys contexthub --proto
$ adb shell incident 3051
$ incident_report 3051

Bug: 146086599
Test: $ incident_report 3051, then compare the content with "dumpsys
      contexthub"

Change-Id: I6c2ed76a1a17f9bfc60d07e68cd66d5eea00e9b0