Mark DataUsageState as nullable to avoid potential NPE risk

mDataUsageState is nullable in UnrestrictedDataAccessPreference.java, so we'd better
add a annotation to getDataUsageState(), and add a null-check before calling it.

Bug: 246828843

Test: manual and Robotest.
Change-Id: If7fea53ec7efd004391a09becdba5709ae686f40
2 files changed