Adds support for predexing libraries to speed up dexing for application

Supported from r21 sdk
the predexed jars should also have jar extension for proper merge of predexed files.
changed the predex jar location to the target directory

Added a flag to enable/disable predex'ing of jars, and its off by default.
Merge pull request #161 from sragu/predex

Adds support for predexing libraries to speed up dexing for application