Set power mode to MODE_FIXED_PERFORMANCE during a user switch.

This reduces average duration of UserController.startUser-fg part
of the user switch by 32% from 269.39ms to 184.65ms.

Bug: 311070231
Bug: 325249845
Test: Run user switch 100 times in a row with flag on and off
Flag: ACONFIG android.multiuser.set_power_mode_during_user_switch DEVELOPMENT
Change-Id: I91c68cbe7dfabdd9dc1d63a9d763040474b02177