Create churn observation use case library for check in reporting.

Library provides the basic functionality and structure used for
reporting the churn observation use case. In a future CL we will add the
remaining implementation of the class methods, and use the library from
the device activity client.

BUG=chromium:1386189

Change-Id: Ibb9d43251fa62cf064d7edfb866e908f7e3de4d6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4233863
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Hirthanan Subenderan <hirthanan@google.com>
Cr-Commit-Position: refs/heads/main@{#1104667}