Simplify handling of nested classes

The `isInnerClass()` is being deprecated as it does not check if the
class is an "inner" class, i.e. a non-static "nested" class it only
checks to see if it is a "nested" class. The latter is what the code
wants but rather than try and synchronize code changes in this with
the Metalava update this change removes use of `isInnerClass()`
altogether and uses `containingClass()` instead of `parent()`. The
former will return non-null only for nested classes. The loop still
has the same behavior.

Bug: 325470146
Test: # Tested by making sure the following are unchanged by this.
      #    flag-api-mapping-PublicApi
      #    flag-api-mapping-SystemApi
      #    flag-api-mapping-ModuleLibApi
      #    flag-api-mapping-SystemServerApi
Change-Id: Ia3bfa101c9f81aacd4d769f09572d34849a6f9a6