Define CellInfo.UNAVAILABLE for Unreported Fields

For all CellInfo classes, there is a common value for
fields that are unreported by the modem: Integer.MAX_VALUE.
To better document this value, this CL defines it as a
constant "UNAVAILABLE" to indicate that for whatever reason
the framework is not reporting this value to the caller.

Bug: 28171234
Test: make; make offline-sdk-docs
Change-Id: I0aa692e3f1b5478284d262ef97fd65b90bce8293