Pref service: add a factory to create the client lib

The client lib is how most code will interact with the service,
rather than using its Mojo interfaces directly.

ConnectToPrefService: This is the main entry point for users.

ServiceTest: Change the destruction order (of the message loop)
to match the creation order.

BUG=654988

Review-Url: https://codereview.chromium.org/2762333003
Cr-Commit-Position: refs/heads/master@{#458912}
13 files changed