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Ephemeral apps were cached when downloaded. The ephemeral app service
managed this cache, uninstalling old apps when the cache got full etc.
It also disabled ephemeral apps when they weren't running.

This change removes most of the ephemeral app service, and replaces it
with a simple implementation that deletes all cached apps at startup.
Some code to disable after they are stopped is left to keep the existing
tests passing.

Once the code to delete all cached ephemeral apps has been in stable
for a milestone, the remaining bits of the ephemeral app system will be
removed.

BUG=517735

Review URL: https://codereview.chromium.org/1415863002

Cr-Commit-Position: refs/heads/master@{#357008}