power: default to sleep after 15 minutes rather than 20 minutes
Beginning with GNOME 3.28, we began defaulting to sleep after 20 minutes in order to comply with some European regulations:
https://eur-lex.europa.eu/legal-content/EN/TXT/HTML/?uri=CELEX:32013R0801&from=EN
See commit 2fdb48fa for details. Anyway, there is a flaw in our plan: even if we tune the setting to 20 minutes, this doesn't actually work in practice for manufacturers affected by these regulations because the system might only begin to suspend after 20 minutes have passed, but they are actually required to finish suspending before this time. Additionally, the timeout won't begin until the system has fully booted into GNOME, which adds additional uncertainty.
Solution: sleep after 15 minutes. This is reportedly what Ubuntu is doing for OEM installs. And it's closer to the Windows default of 10 minutes.