Authenticate with Accounts&SSO only if use_accounts=true

During plugin init and startSync, accounts initialization and
authentication is done only if use_accounts is set to true in the
profile xml

* Added static method tokenFromFile() to read oauth2 token from file
incase auth is to be done locally
* Added member mUseAccounts to indicate whether Accounts&SSO should be
used or not for auth
* Fixed bug related to fetching the last sync time. Check is made to see
if the SyncProfile object is valid after return from ProfileManager
* The flow is fixed to handle the case when auth is to be done locally.
The target filter in this case is set to "local"
* Changed the inheritance from private to public for GContactClient

Signed-off-by: Sateesh Kavuri <sateesh.kavuri@gmail.com>