Add unit tests for BaseInputConnection

Add basic unit tests for BaseInputConnection.
Also some trivial changes in BaseInputConnection:
  - Mark some files with @NonNull & @Nullable annotation.
  - Fix possible NPE in commitContent().
  - Marks public methods for overriding InputConnection interface with @Override.
  - Format file via google-java-format.py.

Bug: 240359838
Bug: 241503197
Test: atest FrameworksCoreTests:BaseInputConnectionTest
Test: atest FrameworksCoreTests:SurroundingTextTest
Test: atest CtsInputMethodTestCases:BaseInputConnectionTest

Change-Id: Idf77d69bcd4d84a48742e0d7a24398df3c0ae7f8