Change from system prop to config

Stop using the system property "ro.sys.time_detector_update_diff" to set
the value that controls how different a suggestion has to be from the
system clock before the system clock is updated.  Use config.xml
instead. System properties are unusual and unneccessary for config like
this.

There are no other references to "ro.sys.time_detector_update_diff" in
our codebase, and none found on the internet, only one mention of it in
our bug tracker by me, so this should be a safe change and easy for
manufacturers to migrate if they are using the system property.

Bug: 282679588
Test: treehugger only
Change-Id: If9d5c77575012c7ec8bb6c5e74e5e80082903254