[Android][3PPWM] Remove feature param use in tests

Tests for the client provider rely on a feature param which is an issue
because:
* cleaning up the feature will break the tests since it's not a param
  that is intended for the launch group,
* it blocks testing options that may change the availability like added
  in https://crrev.com/c/6578128.

The reason for the new bridge is to ensure that the JNI code is only
generated for tests. Not using the JNI annotation causes a compile
error and wrapping them in helpers creates a lot of unnecessary symbols
for testing in production APKs.

Minor changes:
* Adding some test coverage for shared prefs
* Adding some tests which were impossible without JNI helpers
* Wrapped some JNI calls into helpers (helps with follow-ups, too)

Bug: 419497261
Change-Id: I6b97a86af7941d8f030648e283d6b4523eec9dd1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6590633
Reviewed-by: Jan Keitel <jkeitel@google.com>
Commit-Queue: Friedrich Hauser <friedrichh@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1466354}