Minor code clean-ups with no behavior change

As another preparation CL to work on Bug 7254002, this CL cleans up
miscellaneous minor coding style issues that do not cause any
user-visible problems.

 - Add final keyboard to classes that are not intended to be extended.
 - Add static keyboard to nested classes if possible.
 - Fix a typo in log message.
 - Remove a redundant synchronized keyword in method signature.
 - Remove unused class fields.

Test: checkbuild
Bug: 7254002
Change-Id: I3b54179116a74cafc99a3dfd3f2a429a7111c12e