Filter out null bitmaps when showing "Manage Windows" menu

When we have too many instances open,
ActivityTaskManagerService#takeTaskSnapshot returns null. This change
filters out those snapshots so that NullPointerExceptions are not shown
when the Multi-instance menu is opened.

Bug: 380198576
Test: Open more than 50 new windows from header
Flag: com.android.window.flags.enable_desktop_windowing_mode
Change-Id: I25ca5978d642ae7f764d635ecdaf76e2fb6169d7