Update Javadocs for CellInfo Classes

-Add nullability annotations for getMccString
 and getMncString.
-Add IntRange to CellSignalStrength#getLevel()
-Fix a few docstring errors.

Bug: 129902021
Test: make && make update-api && make offline-sdk-docs
      (no functional change)

Merged-In: I61d645f79a23226ab902fc657fe7b07ffd674a0b
Change-Id: I61d645f79a23226ab902fc657fe7b07ffd674a0b