libprocessgroup: Support per-API level task profiles

When task profiles changes happen, devices released under older API
levels might have to use the same profiles as before. This might be
due to for missing kernel features or some other reasons. Add support
for per-API task profiles to accommodate this scenario. With this
mechanism when ro.product.first_api_level is non-empty, the system
looks for /system/etc/profiles/task_profiles_<api level>.json file
and uses it if it exists. If ro.product.first_api_level is not defined
or per-API task profiles file does not exist the system falls back to
the default /system/etc/profiles/task_profiles.json file.
As before, these task profiles can be competely overwritten using
/vendor/etc/task_profiles.json if needed.

Bug: 172066799
Test: boot with per-API task profiles
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Change-Id: I1671b341b3a0fb5e99d0b0788315c61088326b09