[commhistory-daemon] Add LastDialedCache as a hack for Bluetooth

Bluetooth requires the last dialed number from the call log, but for
various reasons can't access commhistory or dbus directly. As a hack,
write the last dialed number from the call log into a file at
$HOME/.cache/last-dialed and keep it updated even when items are removed
from the call log, or the log is cleared.

Sigh.