Add setting for device docked screen timeout.

This new timeout will only apply when the device is docked, while still honoring any maximum timeouts set by DevicePolicyManager or WM.

Test: atest PowerManagerServiceTest
Test: locally on device, modifying the setting manually via adb
Bug: 213908097
Change-Id: I3d992154723fa8871f7c1c24607b16a35c42fb87