Move Trebuchet to default.xml

Even though this is a "custom" app, it is still a core part of android
that needs to be up to date with security patches and QPR updates.
Move it to the main manifest to make aosp-merger work without
hardcoding Trebuchet in the merge scripts.

Change-Id: I1031d90067f4e3f87c93f90f527271c9ad578225