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We add a BorealisAppLauncher class with associated tests, and hook this
class into the BorealisApps service.

This needed minor refactorings of the BorealisContext.

Bug: b/162562622
Change-Id: I3b6bc49b6d494fef4f8a03810048ceb3fd9122b8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2485998
Auto-Submit: Nic Hollingum <hollingum@google.com>
Reviewed-by: Nancy Wang <nancylingwang@chromium.org>
Reviewed-by: Daniel Ng <danielng@google.com>
Commit-Queue: Nic Hollingum <hollingum@google.com>
Cr-Commit-Position: refs/heads/master@{#820638}