Add a host shell function to generate system_server stack traces

For debugging non-ANR weird states in the system server, it is incredibly handy
to be able to force a complete thread-stack dump of that process.  Here's a
shell function that does so (creating the /data/anr/traces.txt file with the
usual permissions if it does not yet exist).

Depends on adb having been configured to run as root.

Change-Id: I01d9d733ef08ffefb00cb61968df36ecf62087f0

envsetup.sh

@@ -687,6 +687,13 @@ function pid()
    fi
 }
 
+# systemstack - dump the current stack trace of all threads in the system process
+# to the usual ANR traces file
+function systemstack()
+{
+    adb shell echo '""' '>>' /data/anr/traces.txt && adb shell chmod 776 /data/anr/traces.txt && adb shell kill -3 $(pid system_server)
+}
+
 function gdbclient()
 {
    local OUT_ROOT=$(get_abs_build_var PRODUCT_OUT)