Remove StubVolume disks upon vold reset events

StubVolumes are managed from outside Android (e.g. from Chrome OS). So,
their disk recreation on vold reset events should also be handled from
outside by 1) listening to reset events, and 2) calling
createStubVolume() for existing StubVolumes on reset events.

Bug: 175281783
Test: m
Test: (Tested in R) Manually induce a vold reset event, and confirm that
Test: 1) vold does not crash, and 2) existing volumes are successfully
Test: mounted again (by calling createStubVolume() for StubVolumes).
Change-Id: I4628eabf809037a547aeef43faedf4dfa57529a6