settings: Add GetApps to AppNotificationHandler

Overrides AppRegistryCache and implements the GetApps
function, used to update the list of apps to be
displayed on the App Notifications Page.

Bug: 1217174
Test: unit_test
Change-Id: I5634b5a9e7408396be2ddf00ddbf17238f5f7a9d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3049512


Commit-Queue: Ethan Mooney <ethanimooney@google.com>
Reviewed-by: Jimmy Gong <jimmyxgong@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Regan Hsu <hsuregan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#908692}