Make update items/photos highlight when they're clickable

- Highlighting used to work not because "stream_item_content" had the proper
background but because it was the default behavior for ListView items.

- Implement StreamItemAdapter.isEnabled() and make all list items disabled,
so they don't highlight when not necessary. i.e. when items don't have a
click listener set.

- Instead set a click listener to the "stream_item_content" view.

- And change the background to "?android:attr/selectableItemBackground",
which is the default background for list items.

- Also make photo views highlightable when they're clickable.

Bug 5180619

Change-Id: I06bff9d1a0e5385fa66edd4577ddefb43587621c