Added logic to check workspace in file transfer plugin

1. Added logic to check workspace in file transfer plugin
2. Added corresponding spec tests
3.
Reference:http://twiki.polyvi.com/twiki5/bin/view/Dev/ProjApolloRuleFo
rPluginDev#FileTransfer
Since 'assets-library://' has already been taken care of in workspace js module, remove platform-related code from FileTransferExec
Move filetransfer workspace-related tests into its own test file 'filetransfer-workspace.tests.js'
Rename 'filetransfer-workspace.tests.js' to 'filetransfer.xface.tests.js'