Only dispatch window visibility aggregation for targetSdk >= N

Some existing apps treat drawable visibility notifications as a signal
to crossfade from a placeholder to the new image for the purposes of
scrolling onscreen via a recycling collection view or similar. Since
dispatchVisibilityAggregated is now called for window visibility
changes and ImageView informs its drawable of the visiblity change,
the extra call triggers a repeat fade-in in some existing apps when
you return them to visibility.

These apps should pay attention to the second parameter of
Drawable#setVisible, which signals that animations should not restart
in response to a visibility change. Updating to targetSdkVersion=24+
will enable the new behavior.

Bug 30216207

Change-Id: I27ce9f09bc7544863f7f7980c273650949db21cc