Add switch to home from desktop test.

- Assert that the desktop app is at the top at start.
- Assert that launcher is at the top at end.

Fix: 418228109
Fix: 418230949
Test: atest SwitchToHomeFromDesktopTest
Flag: EXEMPT adding test
Change-Id: I535ffdb3a1fc234e1323ad94e750cc682a0b05e1