Fix `#getInstance(Context, String, Executor, Runnable)` javadoc

The javadoc stated that a non-existent clientPackageName would return null, when the method would actually throw an IllegalArgumentException, already documented on the `@throws` clause. The error likely happened when copying the javadoc from `#getInstance(Context, String)`.

Change-Id: Ie045eaa1fc2ea707e9ec9810a4b61190bf0a2da2
Test: N/A
Bug: 205124386
Flag: DOCS_ONLY