Use sk_FOO_pop_free instead of sk_FOO_free

The proper way to free a stack of owned "FOO" items is to use
sk_FOO_pop_free since that will iterate through all the FOO instances
in the stack and free them. Calling sk_FOO_free just frees the stack and
not the items.

(cherry picked from commit 64299318644c0c6b86992d414e68d0af236b52c5)

Bug: 10610037
Bug: http://code.google.com/p/android/issues/detail?id=59536
Change-Id: I8af603b10219acb476666e77cc776b6936a19f8d