Add TestApi to force Low Power Standby to be active

Since Low Power Standby depends on multiple factors to activate,
including the private device configuration for the delay after the
device becomes non-interactive, having this TestApi makes testing
faster and more reliable.

Bug: 190822356
Test: atest LowPowerStandbyTest
Change-Id: I5a16083f996bd7421197b08c6632655c8c3d8c29