DexWrapper is now loaded by the SDK instead of the platforms.

DexWrapper is now loaded by the SDK instead of the platforms.

Since tools_r8, dx.jar is now located in platform-tools. instead
of platforms/android-<API>/tools/lib/.

There is therefore no need for all the platforms to load their
own dx.jar through reflection into DexWrapper. The SDK now does it
for all, and the builders query the SDK for the wrapper.

Change-Id: Ia79ccaf95237f2b25b43f26241e6335acc7050a3