Allow kernel thread to read app data files

When vold mounts an OBB on behalf of another application, the kernel
spins up the "loop0" thread to perform the mount operation. Grant
the kernel thread the ability to read app data files, so the mount
operation can succeed.

Steps to reproduce:

  1) Run: runtest --path cts/tests/tests/os/src/android/os/storage/cts/StorageManagerTest.java

Expected:

  1) All tests pass

Actual:

  Test failure, with the following error message:

  loop0   : type=1400 audit(0.0:46): avc: denied { read } for path="/data/data/com.android.cts.stub/files/test1.obb" dev="mmcblk0p16" ino=115465 scontext=u:r:kernel:s0 tcontext=u:object_r:app_data_file:s0 tclass=file permissive=0
  Vold    : Image mount failed (I/O error)
  MountService: Couldn't mount OBB file: -1
  StorageManager: Received message.  path=/data/data/com.android.cts.stub/files/test1.obb, state=21
  TestRunner: failed: testMountAndUnmountObbNormal(android.os.storage.cts.StorageManagerTest)
  TestRunner: ----- begin exception -----
  TestRunner: junit.framework.AssertionFailedError: OBB should be mounted
  TestRunner:    at junit.framework.Assert.fail(Assert.java:50)
  TestRunner:    at junit.framework.Assert.assertTrue(Assert.java:20)
  TestRunner:    at android.os.storage.cts.StorageManagerTest.mountObb(StorageManagerTest.java:235)

Bug: 17428116
Change-Id: Id1a39a809b6c3942ff7e08884b40e3e4eec73b6a