Convert nested_tasks_with_independent_bounds to bugfix

Renames the flag to convert it to a bug fix flag, and moves under DesktopExperienceFlags.

Flag: com.android.window.flags.nested_tasks_with_independent_bounds_bugfix
Bug: 402825303
Test: m
Change-Id: I19cb47428470efc4d9882b97227d220c3bcb5a52