Adds check for license headers and unused imports

[popematt]

Issue #, if available:

None

Description of changes:

I noticed recently that some of our files are lacking a license header. I've added the spotless plugin to ensure that we have a license header on all new java and kotlin files going forward. I also included a check for unused imports in java, but that might be redundant with spotbugs. In kotlin, it's already checked by ktlint.

If you happen to forget to add license headers, you can have this tool add them automatically by running ./gradlew spotlessApply.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.35%. Comparing base (3c1b6b1) to head (051285b).
Report is 29 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #789      +/-   ##
============================================
+ Coverage     67.23%   67.35%   +0.12%     
- Complexity     5484     5506      +22     
============================================
  Files           159      160       +1     
  Lines         23025    23067      +42     
  Branches       4126     4123       -3     
============================================
+ Hits          15481    15537      +56     
+ Misses         6262     6256       -6     
+ Partials       1282     1274       -8     

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