[rilmodem] Queue DTMF requests in driver

Add all DTMF requests to a buffer in the rilmodem driver, and play them as fast as the RIL and modem are able to (i.e. the timing is decoupled from oFono core and D-Bus clients). Also reverts the core change temporarily done via 9aedea4e782c0e1d956acf963d9249301076142b, and adds a DTMF test script.

Signed-off-by: Martti Piirainen <martti.piirainen@oss.tieto.com>