Only relayout when mIgnoredSlots changes

This avoids relayouting sysui when calling add/removeIgnoredSlot has no
effect.

Bug: 190379081
Test: https://ui.perfetto.dev/#!/viewer?trace_id=c57e4aca-d306-66b0-df47-009944328158
Change-Id: I9e5d5c3bfe20c8df10b779e39cdab39ebf95573b
1 file changed