Fix emergency only edge case

When the default sub id doesn't exist and there is only one other
sub id currently, assume that is meant to be the default and use
that to pull emergency state.

Also switch to SparseArray for performance and betterness.

Change-Id: I976de6abb58c46bce0c56793442c0377a39a7071
Fixes: 34119642
Test: runtest systemui