Include manual DND in FakeZenModeRepository by default

This matches the real behavior more closely. Updated tests that were
assuming that the modes list is empty by default, or adding DND
manually.

Updated TestModeBuilder as well so it only has one static (inactive by
default) MANUAL_DND, to simplify references to it and its ID. The main
reason we had both an ACTIVE and INACTIVE version of this previously was
because setActive() used to not work correctly for manual DND, but now
it does, so we can just set it explicitly when needed. Also removed the
static dndMode method because it's cleaner to use makeDndMode and set
fields on that explicitly where needed.

Bug: 382497909
Flag: TEST_ONLY
Test: ZenModeInteractorTest
Change-Id: I939db3a4e22812d0b85705abb2b077e47623cb3b