libprocessgroup: Change per-API level task profiles to override the diffs

Current per-API level task profiles replace the latest task profiles
rather than overriding the diffs. This poses an issue when a new
feature is added which is applicable to older devices as well we
have to change all per-API level profiles. Change the per-API task
profile support to load the latest task profiles first and then override
with per-API task profiles, similar to how vendor task profiles override
system task profiles. This minimizes the changes we need to do to the
per-API level task profiles going forward and is more consistent with
how vendor task profiles work.

Bug: 170507876
Test: boot with per-API task profiles
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Change-Id: I08cc4b374cede7363cf96a30ac96971d49bb7847