Dump connmetrics in protobuf format

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

Currently, "proto" is used to dump the ring buffer in base64-encoded
proto. The newly added "--proto" arg is a dumpsys convention to dump
a service in proto wire format. This change reuses most of the code
for executing "proto". Other cmds remain unchanged.

Command to invoke (any of the following after lunch and env setup):
$ adb shell dumpsys connmetrics --proto
$ incident_report 3049

Bug: 146086778
Test: $ incident_report 3049, then compare the content with "dumpsys
      connmetrics"

Change-Id: Ief2297d99aeb553224a83894ba3b9695ff0b87aa