Allow badging updates to install sessions.

For the system restore use-case, an installer may need to enqueue
their sessions quickly before badging details, like icons, have been
downloaded.  This change relaxes to allow an installer to update
their session badging after the session has been created.  Notify
observers when badging changes.

Rename callback registration methods to match style guide.  Relax
constraint that observers are home app.  Fix bug around internal
progress reporting.

Bug: 17376797, 17389236, 17334199
Change-Id: I5fb88508baea2f08e89a1504fcf5ef972afad4a7