Move Annotations for getCellSignalStrengths

As per API council script's feedback, the scripts
do not support having nullable annotations preceding
the return tupe. Moving the @NonNull annotations to a
script friendly location.

Bug: 126700393
Test: make update-api && make
      && make offline-sdk-docs (no functional change)
Change-Id: Ie063ffa2e311f4eb7d9e07cd5ee212af97d9d45d