Add a Standby option to PowerKey menu

TV requires a Standby entry in Global Actions because the short press
power button will be used to start and stop dream.

Bug: 402759931
Flag: NONE. This new behavior doesn't affect existing code.
Test: atest GlobalActionsDialogLiteTest
Test: Verify on a device with this standby global actions enabled.
Change-Id: I824f2f2d61089ee70ed8ea31ebd8478d9f9c17ec