fix: some static analysis warnings in Java code

[dpogue]

Platforms affected

Android

Motivation and Context

Static analysis identified a few warnings in our Java code. These were the easy ones to fix.

Description

• Add @Override annotations where needed in a test
• Fix improper if condition chaining in PluginManager
• Fix inefficient empty string check in PluginManager
• Don't use boxed Boolean type where simple boolean will do

Checklist

• I've run the tests to see all new and existing tests pass

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.20%. Comparing base (a9a4511) to head (8b1b0a1).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2015   +/-   ##
=======================================
  Coverage   63.20%   63.20%           
=======================================
  Files          24       24           
  Lines        4922     4922           
=======================================
  Hits         3111     3111           
  Misses       1811     1811           

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