Final (?!?!?!) improvements on Activity.dump()

- Fix handle of special args (they were not called unless the app
  targeted T).
- Lazy-adds some dumpables to Activity (so the list of dumpables
  is not created until dump() is called).

Test: adb shell am start com.android.settings && adb shell dumpsys activity com.android.settings --list-dumpables
Test: atest CtsAppTestCases:android.app.cts.ActivityDumpTest

Bug: 149254050

Change-Id: Id25d115d4aef8ab16ceb3d035c134c637e16dadd