The only time `status == null` was when coming from `removeApk()`. By
moving the logic out of `setApkInternal()` into `removeApk()` it makes
it easier to reason about `setApkInternal()` as it now does less. Also,
it was doubling up on the `syncrhonized (appMapping)` and `if (entry !=
null)` logic which is no longer required, because `removeApk()` was
already doing that.
While here, also make explicit the fact that `status` can no longer be
`null`.
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