sources to dex more dynamic

[nenick]

This line should be improved commandLine "${BUILD_TOOL_DX}", "--dex", "--output=${ASSET_DIR}", secondaryFile.get(0).toString(), secondaryFile.get(1).toString()

if you add more files to secondary dex file, then this line need extra secondaryFile.get(XX).toString(). Maybe the command call can be replaced with a copy of the gradle dex task where output ind input is modified.