Replace default showing state to fullscreen in AppListViewFocusTest

This is one of the cls that removes the peeking state in peeking
launcher.

While all transitions to and from peeking state is removed in
production code, it is still valid to directly set the app list to
peeking state in unittests. This cl changed all peeking state
transitions in AppListViewFocusTest to fullscreen state.

This cl also removes redundant setting state to fullscreen, as all
Show() function set the state directly to fullscreen now.

Bug: 1356678
Change-Id: I7b4163460ff9e1cf052fc3924778f263bb71c4e5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3900466
Commit-Queue: Wen-Chien Wang <wcwang@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1047734}