Add LowPowerStandby allowed reason: Ongoing call

When ALLOWED_REASON_ONGOING_CALL is set in the Low Power Standby policy,
apps with an active Foreground Service of type "phoneCall" are exempt
from Low Power Standby restrictions.

Bug: 234002812
Test: atest LowPowerStandbyControllerTest LowPowerStandbyTest
Change-Id: I9ad99130e31c5bbb259dc3a733d57cb413c23df2