Fix back button on multi-column mode.

This CL disables the backbutton handler implemented in
PreferenceHeaderFragmentCompat. Looks like it has an issue that
the handler does not work if it is running in the two-column mode
and fragment stack is empty.

Then, forward the top-left backbutton click to the back button event.

This should be workaround until we figure out the fix in the third_party
library. E.g., even if we fix the back button handler behavior, current
approach in the third_party library looks following a bit legacy manner.

BUG=404074032
TEST=Tryjob. Tested manually on DUT.

Change-Id: I07ef6d017b6d2ecca6e32b2e5e39b75454cab5cd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6978152
Auto-Submit: Hidehiko Abe <hidehiko@chromium.org>
Commit-Queue: Jinsuk Kim <jinsukkim@chromium.org>
Reviewed-by: Jinsuk Kim <jinsukkim@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1520487}