Active camera apps can defy reserved cache space.

We normally prevent apps from allocating into the "reserved" cache
space, but this change makes an exception for an active camera app,
since the user is probably trying to capture an important memory.

This change only lets the active camera app clear up to half of the
reserved space, since we don't want to completely destroy the
experience of all other apps.

Test: manual app before/during/after active camera session
Bug: 38267830
Change-Id: Ie9e63884fb2638ca881e10b894629eea84601648