ANDROID: sched/tune: cleanup schedtune_boostgroup_{init,release}

This update the definition of these two methods to make more clear their
role.

In this refactored version, the slow path entry functions:
   schedtune_css_alloc()
   schedtune_css_free()
are in charge just to allocate and release the memory used to track
schedtune boost groups.

These two functions rely respectively on:
   schedtune_boostgroup_init()
   schedtune_boostgroup_release()
for everything related to setting up data structures and properly
initializing them.

Change-Id: I9336102b5c6a6b5726fd466e99b7d6b28d38f455
Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>