Change BDG from the class to app level?

[daoyuan14]

Pls refer to
https://bitbucket.org/zout/backdroid/commits/ebd246ebfee438c0e27a0c144a8bbab8f472d835
https://bitbucket.org/zout/backdroid/commits/d9d9df4a79f9e686fac23ef1d7a7b894438ff83a
https://bitbucket.org/zout/backdroid/commits/afd4f6d6601710678dad28bbe4b7b143f6a922ac
https://bitbucket.org/zout/backdroid/commits/293315761c249a2b4eae6008f0e5c8d86f5012e3