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This CL fixes some inconsistencies that become larger issues for running
with JS modules and within SWA container.

Issues:
1. The `window.background` was defined using `var background =`, however
for JS Modules this doesn't set in the global `window`.

2. The interface `FileBrowserBackground` is actually used by other apps
other than Files app, despite the prefix `FileBrowser`. Fix
by renaming the interface to `BackgroundBase` and in turn rename the
implementation to `BackgroundBaseImpl`.

3. The implementation `BackgroundBase` doesn't have a `@implements` to
its interface, which was confusingly named `FileBrowserBackground`. Fix
by adding the `@implements` accordingly.

4. The methods `ready()` and `setLauncherHandler()` were the defined in
the wrong interfaces.  Fix by moving `ready()` to
`FileBrowserBackgroundFull` because it's used only in Files app and move
`setLauncherHandler()` to `BackgroundBase` because it's used in all
apps.

No change in behavior, Closure type-checking and current tests cover all
changes.

Bug: 1148545, 1133186, 1113981
Change-Id: I4ca90e826faf88b7def3b12d89ae249c091ec973
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2534551
Auto-Submit: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827162}