Handle stub volumes in StorageSessionController

Currently, StorageSessionController.shouldHandle() returns true only for
emulated volumes and public volumes. As a result, even when we call
MountUserFuse() in StubVolume::doMount(), stub volumes for external
storage (MyFiles and removable media) are not covered with the Android
FUSE layer which is required for features like EXIF redaction.
This CL modifies StorageSessionController.shouldHandle() so that stub
volumes can be properly handled by StorageSessionController.

Bug: 123377807
Bug: 123641356
Bug: 132796154
Test: $ ls -lZ /storage | grep CAFEF00D
Test: $ atest MediaStore_Images_MediaTest#testLocationRedaction
Test: Tested in R
Change-Id: I74caa0c16bd0021c561c599616c53076022aa4de
Merged-In: I74caa0c16bd0021c561c599616c53076022aa4de