Eliminate reflective form factor aware initialization

This CL removes the runtime instantiation of the form-factor specific
SystemUIInitializer via reflection, which is no longer necessary since
Android TV moved to use a dedicated TvSystemUI target.

This reduces the size of the SystemUI.apk by 500 KB by enabling R8 to
discard TV-specific classes.

Test: make
Bug: 282633512
Change-Id: I30cdb0c2a26e631594bb4bd9ddcecd329de10325