Add "pm set-home-activity"

This is needed to change the default launcher on CTS for
ShortcutManager.

- When the default set with DPM.addPersistentPreferredActivity() is
still stronger.

- Ideally we should unify the code with HomeSettings, but I couldn't
find a good way to do this.

- I didn't make it to work to set the default for any intents
because building an intent filter from a command line would be
painful.

Bug 27548047

Change-Id: I66b153c20c7796147f77214b483112629f9d80c5