dx.bat: Don't change directory.

Changing directory to that of dx.bat prevents dx.bat from being
used with relative directories. This leads to all sorts of
issues integrating it with build systems that expect this to
work.

I've tested that this change works under the following conditions:
1. Android SDK in a path containing spaces.
2. java.exe in a path containing spaces.
3. Java object files in a path containing spaces.
4. Output jar file in a path containing spaces.
5. Java object files specified with relative paths.
6. Java object files specified with absolute paths.
7. Output jar file specified with relative paths.
8. Output jar file specified with absolute paths.

Change-Id: I1b47a6cc73169607cc6238acf197e3b467c9bcbc