Convert Cynara, CynaraAdmin and PrivilegeDb classes into singletons

Convert Cynara, CynaraAdmin and PrivilegeDb classes into singletons

These classes are now used by the Service class to perform operations
requested by clients. But they will be also needed by offline client
implementation. Having them as private members of the Service class is no
longer feasible.
To keep their usage simple and available to the client as well, they are
now used as singletons.

Change-Id: I900a368ea14fbe61179c712b6e891f213ca61c5e
Signed-off-by: Rafal Krypa <r.krypa@samsung.com>