task_profiles.json: add taskprofile for dex2oat boot_complete policy

Since installation time is also critical and sometime we want a finer
control on its behavior for vendor. Instead of set dex2oat to background
group, add a new task profile.

The default policy is to mirror SP_BACKGROUND to be compatible with
legacy devices.

Test: Boot
Bug: 188947181
Signed-off-by: Wei Wang <wvw@google.com>
Change-Id: I7d5e113d1c368205f712eadad27775de226131ce