Re-added Dimmer for IME window container

The dimmer was only set for AboveAppWindowContainers so IME couldn't
add a dim layer. Moved Dimmer so all NonAppWindowContainers can add a
dim layer, which includes AboveAppWindowContainers and IME container.
This also fixes the flicker issue from the bug.

Change-Id: I747eb3db4982f820f8a09b7a184d0467d240b335
Fixes: 72645607
Test: Dim layer appears when using search from IME.
Test: go/wm-smoke-auto