Use killUid() instead of killApplication().

killApplication() doesn't just kill the process, but also tears down its
entire task stack, meaning it won't be restarted when that task becomes
visible again. Use killUid() instead, which does have this property.

Bug: 157674306
Test: manual verification
Change-Id: I691d9fe96b64012c96e0595385bb482056b6c4d4