add an API for sending secret dialer code

Telephony provides an pulic API for sending secret
dialer code with the format of *#*#code*#*#.
This API only sent out broadcast for the default dialer app
and is backgound-check compliant.

Bug:33753947
Test: Manual
Change-Id: I10665828e88be07d96c7f2568a0c6878abe2e21b