"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.cpp b/adb/transport.cpp
index 8b3461a..447a8fe 100644
--- a/adb/transport.cpp
+++ b/adb/transport.cpp
@@ -81,6 +81,7 @@
 const char* const kFeatureFixedPushSymlinkTimestamp = "fixed_push_symlink_timestamp";
 const char* const kFeatureAbbExec = "abb_exec";
 const char* const kFeatureRemountShell = "remount_shell";
+const char* const kFeatureTrackApp = "track_app";
 
 namespace {
 
@@ -1175,6 +1176,7 @@
             kFeatureFixedPushSymlinkTimestamp,
             kFeatureAbbExec,
             kFeatureRemountShell,
+            kFeatureTrackApp,
             // Increment ADB_SERVER_VERSION when adding a feature that adbd needs
             // to know about. Otherwise, the client can be stuck running an old
             // version of the server even after upgrading their copy of adb.