Rework PrivilegeDb interface for setting application privileges

Rework PrivilegeDb interface for setting application privileges

While integrating installer code with PrivilegeDb, the existing method for
setting privileges was found to be inadequate. It also would need further
complication to actually do what it was supposed to do.
New UpdateAppPrivileges() method now only updates privileges for application.
To calculate which privileges were added and which removed for the package,
installer will use GetPkgPrivileges() twice: before and after calling
UpdateAppPrivileges(). All three method calls must be done inside transaction.

Change-Id: Ib7e1b8a6b1482c6dcd8b7146c48187797e237bd5
Signed-off-by: Rafal Krypa <r.krypa@samsung.com>