Run search dialog in the system process.

Fixes http://b/issue?id=1905863

This is needed to address two security issues with global search:
http://b/issue?id=1871088 (Apps can read content providers through GlobalSearch)
http://b/issue?id=1819627 (Apps can use GlobalSearch to launch arbirtrary intents)

This also fixes http://b/issue?id=1693153 (SearchManager.OnDismissListener
never gets called)

To fix the security issues, GlobalSearch also needs to require
a non-app permission to access its content provider and launch intents.