adb: move shell service to a separate file.

Upcoming changes to the shell will require significant additions to
the subprocess code, and it will be cleaner if it's in a separate file.

The only functional change here is a new debug tag specifically for
the shell service. Everything else has been copied exactly as-is in
order to make it easier to determine what's changing in upcoming CLs.

Change-Id: I13bd4294059051ee10e0d0c6a06affd8eca62967
diff --git a/adb/adb.cpp b/adb/adb.cpp
index 14df8a1..a6b539f 100644
--- a/adb/adb.cpp
+++ b/adb/adb.cpp
@@ -176,7 +176,8 @@
         {"transport", TRACE_TRANSPORT},
         {"jdwp", TRACE_JDWP},
         {"services", TRACE_SERVICES},
-        {"auth", TRACE_AUTH}};
+        {"auth", TRACE_AUTH},
+        {"shell", TRACE_SHELL}};
 
     std::vector<std::string> elements = android::base::Split(trace_setting, " ");
     for (const auto& elem : elements) {