keymaster: fix authorization set serialization

Invalid and unknown tags were treated as zero size but they where still
counted as entry. This lead to invalid tags being persisted. When
Serialized blobs were used to cache key characteristics, these invalid
tags were send to clients of keystore. However, the serialization cannot
cope with invalid tags.

Bug: 119414176
Test: Successfully used the Skype app which triggered the problem
Change-Id: Ia46ac4a16395db3d10f93d3722eda69d523db478