unit: Add grilunsol signal_strength test
rilmodem: Use grilunsol signal_strength func

Replaced the last rilutil parcel function used to
parse signal strength replies and unsolictied messages,
with the new g_ril_unsol_parse_signal_strength() function.
Also got rid of the unused ril_util_parcel_init() function.
gril: Re-factor g_ril_send to use *parcel

Re-factor g_ril_send to take a *parcel instead of
separate *data and data_len parameters.  This allows
g_ril_send() to free the parcel after it's been queued
which simplified the rilmodem driver code.  g_ril_send()
now also call g_ril_request* to output the tracing log
messages for requests, again simplifying driver code.
Finally removed the unused file grilresponse.h.
rilmodem: Re-factor usage of g_ril_send()

Re-factor usage of g_ril_send(), as it now takes a *parcel
as opposed to *data and data_len.  Also removed calls to
parcel_free() and g_ril_print_request*(),as both are now
handled by g_ril_send().  Finally, got rid of unnecessary
local variables for g_ril_send()'s return value, and similar
change for request (id).
plugins: Re-factor ril's usage of g_ril_send().