Stamp CellInfo with ElapsedRealtimeNanos at HAL

Stamp CellInfo results with the SystemClock nanos
when they are received from the vendor HAL, ensuring
that app users who read this field can interpret the
value against a well-defined reference clock.

Bug: 36971551
Test: atest FrameworksTelephonyTest
Change-Id: Ie1cf0ddc73f38e078022da0342d382ee017a276f