Fix FilesSetup#initializeFile to use temporary directory

As the method was creating files in the current directory, the tests in
libcore.java.nio.file.FilesTest/DefaultFileSystemProviderTest/DefaultFileStoreTest
were failing on CTS as the directory is read-only. Instead, now it
creates a temporary directory.

Also, removed @RunWith annotation from DefaultFileSystemProviderTest as
it is not supported by CTS.

Bug: 30815448

Test: Ran
libcore.java.nio.file.FilesTest/DefaultFileSystemProviderTest/DefaultFileStoreTest
tests on CTS and Vogar

Change-Id: Ieca38f56b4e4dc98c9ffffce92ac15168821cf62