AML: Do not merge launch events on different displays

Generally multi-display is visually independent, so the launch
events on different displays should be tracked separately.

Bug: 123355661
Bug: 135197913
Test: atest ActivityMetricsLaunchObserverTests# \
            testConsecutiveLaunchOnDifferentDisplay

Change-Id: I84e1e800654a440782ff9a85124449ce6d8078ba