Change timestamp unit from nanos to millis

Change getTimestampNanos to getTimestampMillis for both CellInfo and
SignalStrength.
Add documentation to timestamp in SignalStrength.

Bug:148605481
Test: SignalStrengthTest (unit and CTS)
Change-Id: I801b896cf91c0af91bc4c946b2490a9f1fad16ee
3 files changed