[Android] Move low entropy source to Java

Changes how low entropy source is generated on Android. Now, instead
of using base::RandInt, on Android it goes through JNI to query it
from Java.

In subsequent CLs, this low entropy source generated on the Java side
will be used to assign experiment groups to FRE studies that need
the group assignment before the native is initialised.

Bug: 1361404
Change-Id: Ib7421a46238b74ea3c0efaca304da62a385c38ec
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3978211


Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Boris Sazonov <bsazonov@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1072417}