Two modifications to the magnetic target finder algorithm.

First, we ignore FooterView as a magnetic target (b/396204181). Second,
we avoid crossing notification sections when finding eligible magnetic
targets (b/396136526).

Test: NotificationTargetsHelperTest
Flag: com.android.systemui.magnetic_notification_swipes
Bug: 396136526
Bug: 396204181

Change-Id: I2781d73b6d1e547605e7b13333de39a08af5171a