Rename get* Java methods which return boolean to is*

Its more idiomatic in Java for getters for methods that return Boolean to use `isField` rather than `getField`.

No files or tests are named. Search the repository for Java methods beginning with get that return Boolean values, then inspect their callers and any related tests before assessing the full scope. Done means the applicable methods use is-prefixed names, references are updated, and the project tests still pass.

Written by the indexing model from the issue text.