[Android] Do not process NTP scroll updates to the toolbar while in the switcher.
I made a poor assumption that it would not be possible to get to
the switcher while on the NTP after scrolling up to hide the menu
and tab switcher buttons. By long pressing -> open in incognito, you
can then get to the switcher from there. After swiping back to the
normal stack, it would pick up the previously set NTP scroll position
to try and recorrect the visuals. In this case, we can just ignore
updates while in the tab switcher as it will be re-triggered upon
exiting the switcher.
BUG=494881
Review URL: https://codereview.chromium.org/
1164673003
Cr-Commit-Position: refs/heads/master@{#333366}