jni_generator: Use GN metadata rather than build_config.json for java

Now that all targets use srcjar_deps for JNI, use GN metadata to
restrict JNI registration to parsing only files that actually contain
JNI.

* Deletes JniIgnoreNatives annotation
* Deletes |file_exclusions| from generate_jni_registration
* Changes all registration srcjars to be created in only the default
  toolchain.

Bug: 1410872
Change-Id: If7deaf1fa820c655aceae3c8f708d06fd20eb730
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4630912
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Owners-Override: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Sam Maier <smaier@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1160998}