[API feedback] fixed API for NR

This CL included the follow change:
1. Change getPreferredNetworkTypeBitmap to
getPreferredNetworkTypeBitmask [b/126552419]
2. Change setPreferredNetworkTypeBitmap to
setPreferredNetworkTypeBitmask [b/126552419]
3. Add @IntRange annonation to CellIdentityNr.getNrarfcn() [b/126552419]

4. In javadoc, explain the acronym NR to say: (New Radio 5G)
[b/126552559]

5. Mark CellInfoNr.getCellSignalStrength and CellInfoNr.getCellIdentity
    as @NonNull [b/126699572]

6. Mark CellIdentityNr.getMncString() and CellIdentityNr.getMccString()
    as @Nullable [b/126702744]

Bug: 126552419
Bug: 126552559
Bug: 126699572
Bug: 126702744
Test: build
Change-Id: I133075a3688c3fa7550c8a2f43bc13d8b25999b2