Add disk for StubVolume

StubVolume is a volume type for ARC++ external storage. Named StubVolume
because it is managed from outside Android (not through Android kernel).

Previously, StubVolume is a diskless volume. However, as mentioned in
jsharkey@ email, a disk is needed for StubVolume to hold "kInternal"
(external storage type that is "external" from Android perspective,
but is "internal" to the device. For example shared directory from
ChromeOS to Android) and "kIndexable" (whether or not a disk should be
indexed by MediaStore).

The addition of disk means we could expose the createStubVolume API to
add a disk flags, which is also introduced in this CL.

Both kInternal and kIndexable will be introduced in separate CL.

Bug: 132796154
Test: Mount/unmount ARC++ removable device in ChromeOS.
Change-Id: I8b77fa1cf50ab38a2892272154dafdb78f079378