Add aborted start result.

Previously, we were returning START_SUCCESS when ActivityStarter
aborted launching the activity. This hides this activity and makes it
harder to debug.

This CL adds a new start result type to capture this internally.

Bug: 38121026
Test: manual
Test: go/wm-smoke
Change-Id: I97699b22b1eff476724c48db0c29daa0566ad280