Start home activity if focusable task is not available

Remove the condition of stack count. Because there may have
task tiles or tasks of other user.

Bug: 150853749
Test: atest StartActivityTests#testStartHomeIfNoActivities
Test: No black screen by force-stop home package.

Change-Id: Idc02e9fa6a41de190a74b986da6b0a4e09c9e9bb