Notify on parents of nodes that have newly downloaded art

The BrowseTree code currently does a notifyChanged on the node itself
that had cover art added to it. This actually makes no sense at all, as
notifyChanged actually translates (a few levels of indirection later) to
a notifyCHILDRENchanged.

What we really want to do it notify on the parent folder that a child
has changed. This patch does that.

Bug: b/152444243
Test: build, flash, interop with cover devices, atest
BluetoothInstrumentationTests

Change-Id: I3d35368d5856cd3eb3865070bfedf3f863e4ccab
Merged-In: I3d35368d5856cd3eb3865070bfedf3f863e4ccab