Start Home Process as an empty app when killed by LMK.

We maintain the empty app as a persistent process and keep it around
until we go back to home. Once we go back to home, the original
home app behavior persists.

Helps with back-to-home latencies, especially under memory pressure.

Change-Id: I447aeb4c84629c9e3b9d2d4b3095d2b3e43343dc