Change the namespace of enable_coldboot_threadpool flag to system_performance

This commit changes the namespace of an aconfig flag
`enable_coldboot_threadpool` to `system_performance`, as the original
namespace `desktop_pnp` only allows the flag to be rolled out to desktop
builds. As per go/aconfig-namespace, the namespace of the flag cannot be
changed unless the flag is renamed, so it is also renamed to
`enable_threadpool_coldboot`.

Bug: 440470577
Flag: com.android.ueventd.flags.enable_threadpool_coldboot

Change-Id: I8e3e12c4127b3d851c36360c5eab622236cc9880