Add API for apps to declare their "category".

Upcoming platform features need to cluster apps together into broad
categories to help summarize information to users.  (For example,
when presenting battery, network, and disk usage.)

We are tightly limiting the set of categories to keep them easily
presentable to users when summarizing information.  This feature is
not designed to be a general-purpose taxonomy, nor should it be
allowed to become one.

Older apps may not have defined a category in their manifests, so
allow the installing app to define a category on their behalf.

Test: builds, boots
Bug: 33815939
Change-Id: I785b882ee7c18072ef47d56e0fc19ad72888e1b7