jni_generator: Fix parsing of generics. Add tests for overloads

* Generics were not being stripped for return types of
  @CalledByNative.
* Adding overloads to tests adds coverage of name mangling logic
* Fixes error handling logic of ParseError

Bug: 1406605
Change-Id: I3d977d1179f16d1bae35a628f578ec0a75d1aff7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4655003
Auto-Submit: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Sam Maier <smaier@chromium.org>
Commit-Queue: Sam Maier <smaier@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1164716}