Android: Compile jar file on Windows

Due to the way the DEX_CMD is formatted on Windows this would break
every time. Since we actually bundle dx.bat in the repository, there's
no need to check for its existence, so the easy fix is just to move
the existence check into the code path where it's run from the SDK.

Task-number: QTBUG-31405
Change-Id: If1aeb744d3abbd2488153b13aac401436965074e
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Ray Donnelly <mingw.android@gmail.com>