CellTower: de-duplicate identity string creation, add alternate identity

Signed-off-by: mvglasow <michael -at- vonglasow.com>
CellTower: return non-null identity for cells with partial information

Signed-off-by: mvglasow <michael -at- vonglasow.com>
Support alternate cell identity (PSC/PCI)

Eliminate duplicate cells from display
Use setter methods in constructors of CellTower subclasses
Introduce cellTowerList.getAll() (list of entries without duplicates)
Handle duplicate records in cellTowerList.removeSource()
Get entries from CellTowerListGsm by PSC, from CellTowerListLTE by PCI
Handle alternate identities in CellTowerList.update()
Update existing cell info in CellTowerList.update()
Fix bug in CellTowerGsm.setLac() and CellTowerLte.setTac()
Set PCI in CellTowerLte constructor

Signed-off-by: mvglasow <michael -at- vonglasow.com>