Check when trust agents do not declare a settings activity.

Trust agents are allowed to optionally specify a settings Activity in the
meta-data file. This CL fixes the logic that parses the file to ensure that a
missing settings activity does cause a NullPointerException.

Bug: 133557168
Test: Registering a trust agent without a settings Activity works
Change-Id: I0736a105fee77032acf2ac4b098717b6d49b4099