Add StorageManager Test

Bug 4268904

Adapted from frameworks/base/core/tests/coretests/
src/com/android/server/MountServiceTests.java

The test used some private FileUtil APIs. CTS already
has FileUtils but under the tests/permissions directory
and not for com.android.cts.stub. Copy the FileUtils
implementation from tests/permission and add the
methods used by the test. The OBBs were copied and
their signatures were updated to be owned by
com.android.cts.stub.

Change-Id: I67ebd52fc7e7c97c5b93609754d9c2a5d5165196