Improves dump() to handle special cases.

When "dumpsys activity" is called with special arguments (like
--list-dumpables), dump() should be handled by Activity itself,
not the subclasses, otherwise the end result would be spammed.

This CL fixes that behavior, but only for apps targeting T (so it
maintains backward compatibility for apps that are explicitly
checking for the special values).

Bug: 149254050

Test: m update-api
Test: manual verification with 'adb dumpsys activity ...'
Test: atest CtsAppTestCases:android.app.cts.ActivityDumpTest
Test: adb shell dumpsys platform_compat|grep DUMP_IGNORES_SPECIAL_ARGS||echo NOT_FOUND

Change-Id: I918c9a6649cb66f9e2b6509a643005fac110d152