Module support for -javaagent arguments to dex command

Added option for module developers to specify a property name of "dexAgent" with a jar file name in their timodule.xml. This allows for prepending the dexArgs in the runDexer function with "-javaagent:", which is necessary for some modules.

@cb1kenobi @ingo

We tried using the build hook system, but it does not allow for the correct placement of "-javaagent" in the dexArgs.  This is a generic solution for any module developers who need to add -javaagent.  They put the .jar file in their modules directory add this line to their timodule.xml:
<property name="dexAgent" type="string">class.foo.jar</property>