[Lacros] Clean up LacrosExtensionAppsController::Launch().

LacrosExtensionAppsController::Launch() originally handles Chrome v2
Apps only, and we plan to extend it to handle Extensions opened by
File.app via fileBrowserHandler API. To prepare, this CL cleans up
LacrosExtensionAppsController. Details:
* Fix headers.
* Extract FinallyLaunch() to perform launch after preconditions are
  satisfied. Our upcoming change will change this extensively.
* Launch(): Previously if IsAppLaunchableWithoutEnabling() returns
  false, then the flow enables app asynchronously, and on completion,
  calls FinishedEnableFlow(), which then calls Launch() to retry. This
  "async recursion" is not robust. Since FinallyLaunch() is available,
  so the flow is redirected there instead.
* Create LaunchResult once in Launch() and pass it around.
* Extract lacros_extensions_util::DemuxId().
* Change LacrosExtensionAppsController to call DemuxId() instead of
  DemuxPlatformAppId(), since the class will handle Extensions now,
  in addition to Apps.

Bug: 1275654
Change-Id: I2c1cff86e211cd10a192fb5a6142b8aa85ca6033
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3543830
Reviewed-by: Erik Chen <erikchen@chromium.org>
Commit-Queue: Samuel Huang <huangs@chromium.org>
Cr-Commit-Position: refs/heads/main@{#984135}