Add platform infrastructure for features.

This introduces a new mechanism to define features associated with
a platform, query the current device for the available features,
and enforce that apps requiring features that aren't available can't
be installed.

Also now allows uses-library to specify that a library is optional,
so the lack of such a library will not prevent the app from being
installed (but if it does exist it will be correctly linked into
the app).

Change-Id: I5b369b46cfa0b3d37c9e08fd14ef1098a978e67b