[App Management] Make TestStore generic.

This CL makes the bookmarks TestStore generic, so that it can be
extended to create test stores for different store implementation
classes.

Bug: 906508
Change-Id: I80831d93c747e7b8964e3630719549277b9a0618
Reviewed-on: https://chromium-review.googlesource.com/c/1372969
Commit-Queue: Reka Norman <rekanorman@google.com>
Reviewed-by: calamity <calamity@chromium.org>
Reviewed-by: Eric Willigers <ericwilligers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616573}
4 files changed