Test Legacy Storage on public volumes

Add a new test class to CtsScopedStoragePublicVolumeHostTest
The new test inherits from the existing LegacyStorageTest and runs all
of the test cases there but on a public volume.

In addition, refactor some of the utilities to make that possible.

Test: atest CtsScopedStoragePublicVolumeHostTest
Bug: 148841336

Change-Id: If01085024becb4d0a534f79834d2339e07991a9c
7 files changed