Fix IOOB when stabilizing bundle group children

Fixes: 417981398
Flag: com.android.systemui.notification_bundle_ui
Test: manual
  1. have existing group inside of a bundle
  2. open shade, expand bundle
  3. receive new child notification in group
  Observe: no crash
Change-Id: I455ffbfdcb59e0f5ba34d8b3700fcf6480f70b69