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If an interface has only one implementer, R8 will merge the interface
with the implementer. This causes problems if DexSplitter moves this
class into a DFM, as the parent dex may have a reference to the
interface, which is now a reference to the implementation in the DFM.
This will cause a LinkageError at runtime, since the parent ClassLoader
will try to load the reference to a class found in the child
ClassLoader.

I timed the proguard step for monochrome_bundle on my machine locally,
the whole thing takes about 100s while _VerifySplitDexFiles takes about
3s. The verify step will only be run for bundles with isolated splits
enabled which use the chrome module (monochrome/trichrome chrome
bundles).

Bug: 1154065
Change-Id: I453f10354ca20f355c8a946ed611a9ff186ab50f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2567476
Commit-Queue: Clark DuVall <cduvall@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#832657}