Remove the "expand/collapse" actions on the apps list in 2 pane picker.

In a single pane picker, apps list is expandable / collapsable, but,
its not applicable for 2-pane variant. in 2-pane one, it should just
say "double tap to activate".

Also opened b/324073588 to make it clear from a11y perspective that it
is two pane and on click, right pane change should be announced.
(just like it does in settings with talkback).

see screencast/cast/NTQ1Mjk0MjIzOTg1ODY4OHw4OTc5ZjAzNS0wOQ

Bug: 319195592
Flag: None
Test: WidgetsListHeaderAccessibilityTest & manual.
Change-Id: I9ae3fbacc29364bd7459f6e6d85e0a1e919de3f8