Force activities to run on the second display for testing.

This is a simple hack for testing and development purposes.
It makes the framework place the main window of an activity
on to a secondary display instead of on the default display.

Set the "debug.second-display.pkg" to a substring of the
package name of the activity that you want to have show
up on the secondary display, such as "com.example.android.apis"

Bug: 7183618
Change-Id: I0a9e7f27c8ff253253b9de57d4bc49f31d95a0e2