Move power button cooldown period flag to Settings.Global

This is a follow up of ag/16406749

Bug: 195782998
Test: unit tests, manual test on device
Change-Id: Icb977d9a7591ffe48ae44bbe2ad521945b7b0139
Merged-In: Icb977d9a7591ffe48ae44bbe2ad521945b7b0139
(cherry picked from commit 5ab84660)