Android: Switch cast_shell to use implicit JNI registration

Explicit registration is needed only when using the custom linker.
Cast shell doesn't use the custom linker, so has no need to jump
through the extra hoops involved with explicit registration.

Change motivated by JNI refactorings, which are made simpler by having
fewer targets use manual JNI registration.

Bug: 898261
Change-Id: I88e92480d355fc0a19ffdd4c816efbf749dd9964
Reviewed-on: https://chromium-review.googlesource.com/c/1317340
Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Commit-Queue: agrieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605446}
3 files changed