Fix Automated API Review issues.

1. Some API's argument/return value must be marked either
@NonNull or @Nullable.
2. Change some system APIs to public APIs.
3. Modify the method name

Bug: 126700123
Bug: 126702339
Bug: 126699682
Bug: 118296575
Bug: 126699216
Bug: 126699675
Bug: 126699429
Bug: 126699193
Bug: 123586045
Test: atest FrameworksNetTests
Change-Id: Iaa2832cdcf83758ed0fec81b954a0c63bc5a7bf6