Handle enter key as editor action in search bar

Handles IME_NULL action in AllAppSearchBarController, which is generated
in response to the enter key (with no modifiers) keyboard event in
multiline text view. The action triggers search in the same way as
ACTION_SEARCH and ACTION_GO. Without this, enter key gets treated as
new line in the app list search box.

Test: Open all apps, and enter a search query using physical keyboard.
     Pressing ENTER triggers search. Pressing Shift + ENTER adds a new
     line to search query.
Bug: 370780257
Flag: com.android.launcher3.multiline_search_bar
Change-Id: I6aef5e120210f4efc216e03d24974b04ab5f1d16