Move code causing verify error into separate helper class
Fixes#748.
I'm not 100% sure on how the `@TargetApi` and `VerifyError` work
together. However it is something along the lines of:
* Class loader needs `CleanCacheService`.
* At this point, it loads the bytecode for that class and verifies
that it all makes sense.
* The bytecode within the method targeted at API 21 is not understood
by earlier APIs, because the entire `Os` class was introduced in 21.
* By putting it into a different class, that class is only loaded
at runtime on devices with API of 21 or higher.
Previously, `@TargetApi` + the relevant guard condition to check
the build version at runtime suffices to prevent this. However it seems
that if the entire class does not even exist on earlier APIs, then it
is no longer good enough.
See merge request !379
5a6aecad85