[sdk-platform-java] License enforcer for modified Java files

Thanks for stopping by to let us know something could be better!

**Is your feature request related to a problem? Please describe.**
Google Copyright should be written similar to `Copyright 2010 Google LLC`. Currently, a lot of our existing code has copyright enforced as `Copyright {Year} Google Inc.`.

When we make a code change, we should enforce that the header should be updated to `Copyright {Original Year} Google LLC`

**Describe the solution you'd like**
When there is a modification to the Java code, we should have a check to ensure that the header is modified to the correct value.

**Describe alternatives you've considered**
Could the checkstyle plugin be enhanced so that it is enforced on updates? Or can we write a script to check for modified files and check that the copyright is being updated to LLC?

The current checkstyle plugin only checks that it follows the regex in `java.header`. Since we have whitelisted either `Google Inc.` and `Google LLC`, it is currently not being enforced.

**Additional context**
For more information, see the internal page about copyright.