[ExpandableNotificationRow] use shouldShowPublic in isExpanded

mShowingPublic is only updated from SSA after setting its height.
This could make its value stale during height calculation and cause squishness.
This CL uses its version that is updated before shade invalidations.

Bug: 324537506
Fixes: 332307645
Flag: NONE
Test: manual, steps are in 332307645 and 275457408.
Change-Id: I88bb6da926417a4d8e2bd4d31b70b87296cac07f