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_trace.h b/adb/adb_trace.h
index 67ee854..623108e 100644
--- a/adb/adb_trace.h
+++ b/adb/adb_trace.h
@@ -37,6 +37,7 @@
     TRACE_SERVICES,
     TRACE_AUTH,
     TRACE_FDEVENT,
+    TRACE_SHELL
 };
 
 extern int adb_trace_mask;