Browse mode for DocumentsUI, removed volume state.

The existing management mode is too specific, and requires that
storage backends add queryChildDocumentsForManage(), etc.  Instead,
to offer more natural browsing support, add a new BROWSE_ROOT intent.

It behaves mostly like MANAGE_ROOT, except that it doesn't mutate
its Uris with setManageMode(), and it shortcuts straight to VIEW on
clicked documents.

It can be launched like this:

$ adb shell am start -a android.provider.action.BROWSE_ROOT
    -d content://com.android.externalstorage.documents/root/8405-1DFB
    -c android.intent.category.DEFAULT

Also rename a MetricsConstants to make it clearer, and don't
auto-mount all emulated volumes.

Fix bugs around parceling of DiskInfo/VolumeInfo.  Method to resolve
the best description for a VolumeInfo, which might need to fall
back to DiskInfo.

Add back "removed" volume state so we send broadcast when a volume
is destroyed, matching the expected public API behavior.

Bug: 19993667
Change-Id: I13aff32c5e11dfc63da44aee9e93a27f4690a43f