Use maxBy instead of max for UnusedPeriod enum

Replace max with maxBy in UnusedPeriod enum in UnusedAppsViewModel. It makes the code less error prone in case out of order element is added to the enum.

Relnote: Improving user info message about Unused Apps Hibernation.

Bug: http://b/265670665
Change-Id: Ie6eb7a78458767838aea90630d173678995a85e7
Test: Locally on TV with m -j50 GooglePermissionController
Followup: I4165b840ec38b53a6bc5290fa778a0409a5986a3
(cherry picked from commit 91dedc52)
Merged-In: Ie6eb7a78458767838aea90630d173678995a85e7
(cherry picked from commit ed075dcb52f184964df3442a3c546c349fb273be)
Merged-In: Ie6eb7a78458767838aea90630d173678995a85e7