Add a flag to allow using all CPU cores during a user switch.

System UI is not allowed to use the big CPU cores to reduce battery
usage. But during a user switch we should use those resources to make
the switch faster. This flag will enable that modification.

Bug: 308105403
Test: m all_aconfig_declarations
Change-Id: Ie62ca9af18e9a4b4ac41dadb6c54cc727218e4bc