Allow user processes to query current memory level over D-Bus

The get_memory_level method call was missing from mce dbus configuration
file and thus non-root processes could not query the current state.

Add dbus config entry for the get_memory_level method call and allow
non-root processes to make the query.

[mce] Allow user processes to query current memory level over D-Bus. Fixes JB#24382
Merge pull request #229 from spiiroin/jb24382_memnotify_dbus_query

Allow user processes to query current memory level over D-Bus