Test Scoped Storage on public volumes

Add a new CTS test: CtsScopedStoragePublicVolumeHostTest
The new test inherits from the existing ScopedStorageTest 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 --rebuild-module-info
Bug: 148841336

Change-Id: I933a4e32cf09579c8906cf1f25e41da773b62a4c
9 files changed