Have MediaScanner operate on MediaVolume.

Instead of using volume names, use MediaVolume objects to keep track of
what volume we're scanning.

Bug: 182396009
Test: atest CtsScopedStorageDeviceTest
Test: atest MediaProviderTests

Change-Id: I35588ed446c7f4a3f5c335af966359e1e266b62f
11 files changed