Fix ArrayDeque.clone on andriod-8

`Arrays.copyOf` was introduced in API 9, so the `minSdkVersion`
of 8 is not enough. Substituting for `System.arraycopy` makes a
bit more sense at this point in time.

See https://github.com/koush/AndroidAsync/issues/2#issuecomment-21034492 for discussion.
Merge pull request #37 from soulseekah/patch-api-8-compa

Fix ArrayDeque.clone on andriod-8