[Android][Passwords] Map JNI tasks to backend replies

Java can't hold or call C++ callbacks via JNI (as far as I know).
Therefore, this CL introduces a mapping from TaskIds to callbacks that
are called upon creation of a JNI task.

Since each Task only needs a callback, a variant allows to store exactly
the one that will be called afterwards. It is most-likely that the data
filtering happens on the java side and therefore, other parameters won't
be necessary.
Other native post-processing steps can be bound as callback as well.

Bug: 1229654, 1229655
Change-Id: I483469e91b159d4aa38d0baf1114c7a52d32530b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3046191
Commit-Queue: Friedrich [CET] <fhorschig@chromium.org>
Reviewed-by: Vasilii Sukhanov <vasilii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#907099}