dex2oat: add a new task profile 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 setting dex2oat to
background group, add a new task profile (by default same as background
group).

Test: Boot
Bug: 188947181
Signed-off-by: Wei Wang <wvw@google.com>
Change-Id: I37a36050a338f9eba8d605a00a32b99a4a590bac