"track-app" service showing debuggable/profileable apps

Add a "track-app" service in adbd. For every debuggable or
profileable-from-shell process, ART sends related info to
adbd and adbd surfaces the info through the "track-app"
service.

The output format of "track-app" is a line summarizing
the number of reported processes, followed by a protobuf
message in human readable form. For example,

Process count: 2
process {
  pid: 3307
  profileable: true
  architecture: "arm64"
}
process {
  pid: 3341
  debuggable: true
  profileable: true
  architecture: "arm64"
}

Bug: 149050485
Test: manually unplugged/replugged, "adb track-app",
      "adb track-jdwp"
Change-Id: Id1f1a920e1afc148c7e4d2add790baab796178e1
diff --git a/adb/transport.h b/adb/transport.h
index 8a0f62a..a62349e 100644
--- a/adb/transport.h
+++ b/adb/transport.h
@@ -82,6 +82,8 @@
 // adbd properly updates symlink timestamps on push.
 extern const char* const kFeatureFixedPushSymlinkTimestamp;
 extern const char* const kFeatureRemountShell;
+// adbd supports `track-app` service reporting debuggable/profileable apps.
+extern const char* const kFeatureTrackApp;
 
 TransportId NextTransportId();