Add error detection for missing recent tasks list in SystemUiProxy

There are ways for launcher to lose the recent tasks reference, in which case SystemUiProxy will quietly fail to start the recents activity.

Flag: not needed
Fixes: 301257750
Test: programmatically forced the error condition and checked the logs
Change-Id: I1cad6a3dfd2605f275672d1dcd8c83bdb965d73f