Filter volumes to only those the caller can see.

Don't return volumes that caller can't actually use.  Also add feature
to indicate that device supports "virtual disk" feature to verify
secondary external storage behaviors.

Bug: 117932814
Test: atest MediaProviderTests
Test: atest cts/tests/tests/media/src/android/media/cts/MediaScanner*
Test: atest cts/tests/tests/provider/src/android/provider/cts/MediaStore*
Change-Id: I66091c532ab5dac5032a82b136c590ac5a2a983a