Add a method to query if an app is installable in the Play Store.

This CL adds a new method on app.mojom that allows Chrome OS to query
ARC for whether a given package name is currently installable.

BUG=969560

Change-Id: Ia525a92944b9b4afb4fd5f6a1ccb150e931a22ca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1640917
Reviewed-by: Yury Khmel <khmel@chromium.org>
Reviewed-by: Yusuke Sato <yusukes@chromium.org>
Commit-Queue: Dominick Ng <dominickn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#666171}