Added a settings for CMAS message broadcast

This is for sl4a automation to receive CMAS messages
broadcasdt. The automation should inject the settings
through 'adb settings'.

Test: manual
bug: 34393649
Merged-In: Ic29be11b641f185219146d7fbb05561f5907ea76
Change-Id: Ic29be11b641f185219146d7fbb05561f5907ea76