attempt fix for adding option for sigterm

[themiswang]

Test Steps:

Enable case:

• Adding FirebaseCrashlyticsSIGTERMEnabled=YES to Info.plist
• Build app, launch without debugger attached
• Run kill -TERM <pid>
• Seeing signal record

Disable case:

• Removing the flag or set the FirebaseCrashlyticsSIGTERMEnabled=NO
• Build app, launch without debugger attached
• Run kill -TERM <pid>
• Don't seeing signal record anymore

no-changelog

[themiswang]

Close this pr for now as there is more needs on product discussion.