`FileUtils` and `AtomicFile` under Ravenwood.

These are common utility classes used by framework code, so support
them and their relevant tests.

Since they depend on ErrnoException and IoUtils, bring over limited
versions of those classes into `libcore-fake`, and compile them into
the Ravenwood runtime using variant washing.

Bug: 292141694
Test: atest FrameworksCoreTestsRavenwood FrameworksUtilTestsRavenwood
Change-Id: Ib44dedacfa4d12a7d697973caf070477ed07202d