* New method in AttachmentProvider to delete all attachments for an
account, and unit test it.
* New method in Controller to delete all synced data for an account,
and unit test it.
* Fixed existing problem with Controller unit tests (needed cache clear)
* Fixed existing problems in Controller (mContext vs. mProviderContext)
Bug: 3197935
Change-Id: I79c6a03c21f18d37eeb8158cd1c2af0e0a6e9d2e