Replace collect with clear for the SoftReference policy.

Replace collect with clear for the SoftReference policy.

The garbage collector is invoked with a parameter that changes the
SoftReference clearing policy.  This parameter is normally false,
meaning the garbage collector will attempt to preserve some
softly-reachable referents of SoftReference instances.  When true, the
garbage collector will treat SoftReference instances as WeakReference
instances and always clearly referent fields with softly-reachable
referents.

The code refers to this action as collecting soft references which is
misleading.  The change to the garbage collector policy has an effect
on the referent field of SoftReference instances and any SoftReference
instance which is unreachable is subject collection like any other
object.  With this change we now use the description clearing soft
refernces to describe the policy.

Bug: 3381480

Change-Id: Ie179514e68d4621237b08658c1c55811a49f1210