Add a ContentProvider for allowing remote-application access to KeePass databases

As I've written it:

# The client application must open the KeePass database itself (it sends database/password/keyfile to the ContentProvider's open() method)
# If the current client uid == the starting client uid: access is automatically granted
# If the uid is not the same, an access prompt will be shown. Choices are Deny, Allow and Always. 'Always' gets reset upon app changes (e.g. updates)

Clients should copy `com.keepassdroid.provider.Contract` locally to make use of the ContentProvider I have created.

I will be publishing an app that will make use of this new ContentProvider soon and would like to see KeePass have this support by then  :-)

Future changes to the ContentProvider that I might implement would be icon fetching support, for rendering entry icons in the local client.

There is a very trivial test case included with my pull request.