Fix handling for onItemClick of fixed collection items

The onItemClick handling code assumed that the item would be nested at
least one layer deep due to the RemoteViewsAdapter adding a wrapper view
group.

Rather than add another case to that logic, I've refactored this to
traverse the view's children looking for a view with the tag. As the
tag is internal, there should only ever be one child with it and we'll
always want that one to handle the click.

Fix: 190353630
Test: locally
Test: atest RemoteViewsFixedCollectionAdapterTest
Change-Id: I22057f148d33482ad84fff592b9f7f554fa2bfad