Add metric to record Crostini App Launches

For each Crostini app launch, this records the type of app being
launched - either the terminal, a registered app, or an unknown app.

Bug: 703927
Change-Id: I326ff4a11028b1030b3c86a0ea4e98048441b983
Reviewed-on: https://chromium-review.googlesource.com/1025542
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Reviewed-by: Timothy Loh <timloh@chromium.org>
Commit-Queue: Ben Wells <benwells@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553515}
3 files changed