Mahi: Set up MahiManager API

Creates functions within MahiManager that will be used in the UI to get
the content from the backend.

Bug: b:319329379
Change-Id: I91c111d7e211332f22351dceb0b11bce90930aa6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5308574


Reviewed-by: CJ Huang <chenjih@google.com>
Reviewed-by: Thanh Nguyen <thanhdng@chromium.org>
Commit-Queue: Andre Le <leandre@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1263514}