Enable interception of more filesystem APIs.

Previous changes have let us open() and stat() paths, but we also
discovered that we need to support rename(), unlink(), and remove().

Also change UnixFileSystem to perform operations through Libcore.os
to give ForwardingOs a chance to intercept them.

Bug: 124349250
Test: atest CtsLibcoreTestCases
Test: atest android.appsecurity.cts.ExternalStorageHostTest#testExternalStorageRename
Change-Id: I289a710880063057a908a593294a37d7231f08ff