Introduce a grace period for bg activity starts

We allow processes that either had an activity started in them
recently, or recently finished an activity while they were
participating in a visible task to start activities from background
within 10 seconds.

Bug: 132333960
Test: atest WmTests:ActivityStarterTests
Test: atest CtsWindowManagerDeviceTestCases:ActivityStarterTests
Test: atest BackgroundActivityLaunchTest
Test: manual with bugs on b/hotlistid:1573032
Change-Id: Ica2c1abecf881ff9bb0959f17456d816350e4e96