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The launch handler follows the approach of `setCurrentDirectory`, but
it is much simpler. There's no need to obtain the File in order to get
the MIME type, since we assume all selected files are "related". (The
Files app does not offer the media app as a viable launch task if all
selected files do not match the file handler type declaration).

Update and consolidate the test helpers used to trigger launch. Expose
the "real" launchConsumer and use it for existing tests as well for
extra test coverage.

Bug: b/159107125
Change-Id: I2c0239b58ba9ae0dfe472b600af086a941c828ce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2247922Reviewed-by: dstockwell <dstockwell@chromium.org>
Commit-Queue: Trent Apted <tapted@chromium.org>
Cr-Commit-Position: refs/heads/master@{#779651}