Remove isSticky true for SplitPlaceholderRule

SplitPlaceholderRule#isSticky determines whether the placeholder
will show on top in a smaller window size after it first appeared
in a split with sufficient minimum width.

This change will show Settings homepage while users rotate
device to portrait mode and show Settings 1-pane UI.

Bug: 256115335
Test: manual visual
      1. Set font size and display size to large.
      2. Launch Setting app in landscape and rotate to portrait.
Change-Id: Ie955f8b8db3bbe42dba6300608774582ef150590