SignalStrength: Allow for custom signal buckets in LTE

STRICT levels are too low, LENIENT levels are too high. Let us
use something in between...

To make things more fun, getAlternateLteLevel() used yet another
set of values. Subsume all of this functionality into a main
getLteLevel() function that uses whatever the device has configured
as the signal limits, and wrap the old methods around it in
case we additionally need to force RSRP for all cases. getLteLevel()
without arguments and getAlternateLteLevel() retain the same
functionality.

Change-Id: I134774ed471e1805eac95cfbf8af957378fd8293