OsTest: Only test sendfile() API, not separately the implementation method.

Testing the implementation method is inconsistent with the other methods
tested by OsTest, and offers little value because (a) it is not part of any
API surface and (b) Android's implementation in android.system.Os.sendfile()
directly delegates to it.

That the other version was ever tested is a historical accident: Before
http://r.android.com/937425 (bug 129523169), generally only the *internal*
version of each of these methods was tested. When the test for sendfile()
was added, the need to test the actual public API surface was recognized
but the code also wanted to stay consistent with the rest of the test, so
coverage for both was added. Now that the other OsTest methods have moved
to testing the public API, there is no longer a consistency reason for
also testing the private API.

Test: atest CtsLibcoreTestCases:libcore.android.system.OsTest
Change-Id: Iacb32601f2714a50172b79c14231a0b1a26a70a9