Add dexmerger support for Windows

This CL adds a dexmerger.bat script that uses the
already present dx.jar file in order to be able to
build tests that rely on this tool, namely the smali
tests.

Bug:

Change-Id: I4b69f6b60c7fc7e042b82c36657e5fcf22a0239a
diff --git a/tools/windows/README.dx b/tools/windows/README.dx
index 99db256..348dc4a 100644
--- a/tools/windows/README.dx
+++ b/tools/windows/README.dx
@@ -21,3 +21,5 @@
 ---
 > call java %javaOpts% -Djava.ext.dirs="%frameworkdir%" -jar "%jarpath%" %params%
 
+dexmerger.bat has been copied from dx.bat, and the command line has been updated
+according to the SDK dexmerger bash script to call the right class.
\ No newline at end of file