Extracts NotificationActivityStarter from NotificationPresenter.

This change simplifies the NotificationPresenter interface by extracting
the methods dealing with starting an activity due to actions taken on a
notification to a separate interface, NotificationActivityStarter.
The extracted methods in StatusBarNotificationActivityStarter are also
cleaned up to make the control flow easier to read.
Pure refactor; no functional changes.

Test: atest SystemUITests
Change-Id: I2e8cf0626a005dc52b9969865a097cc14da9b669