A11y fix for all widgets tray (Part 1 / n)

Mark some image view unimportant for accessibility.
Still need to update the content description for widget list
header to describe the expand / collapse action.

Test: Navigate all widgets tray with talkback enabled.

Bug: 183120145
Change-Id: I1801d8351ac7f22a0135a96a36de706377781b31