Revert "Revert "Default 'persist.sys.timezone' property to GMT if not set on init.""

This reverts commit 27c461ab.

I'm taking alternative approach, telephony code will treat "GMT" value as it would be unset. This is fine because GMT is a synthetic value that would never be set from other sources. 

Change-Id: I889a844bfd75b7730a5d967065d27de03ef3ce2f
Test: flashed device, tested manually 
Bug: 64056758