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Fakes module install by copying a feature module on the device's
download folder into the directory where SplitCompat expects downloaded
modules. Then invokes SplitCompat to emulate the module. The fake
install mode is activated via a command line flag. The fake install mode
is useful for testing purposes where it is difficult to host the module
on Play.

+ Refactors install logic out of ModuleInstaller into the
  ModuleInstallerBackend.

+ Adds two backends - one for the existing install logic and one for the
  fake install logic described above.

Bug: 862702
Change-Id: I8f2deff34a9aa4f8c0d434429fe7895038be7464
Reviewed-on: https://chromium-review.googlesource.com/c/1308608
Commit-Queue: Tibor Goldschwendt <tiborg@chromium.org>
Reviewed-by: agrieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604393}