frameworks: fix setting default ringtone for multi sim

The defualt value for the ringtone expects a "_" between the setting and
the requested sim subscription. This was not being respected in the
mediascanner when it set the default ringtones, resulting in the setting
value being "ringtone2" instead of "ringtone_2".

Change-Id: I4a9147d579183229cc3300e71194e6f5ab74fbc6
Signed-off-by: Roman Birg <roman@cyngn.com>