Revert back to a single home app in mHomeProcess

The idea of multiple processes serving as home was unfeasible.

- Revert "Allow for more than one home app." commit
e428a7f6.
- Assign ActivityManagerService.mHomeProcess to the process of the
root activity of the home task.

Addresses bug 10342471.

Change-Id: Ifb494626107d24de1306e320a18206d5b176a7c0