SystemUI: fix missing rotate suggestion in 2 button nav mode

"contextual", "menu_ime" and "right", we get three different variants
for one and the same thing. Or well, that's how it was, until Google
decided to break it. "contextual", which was used for 2-button nav due
to a Google change, now contains the floating rotation button, but the
code expects the in-navbar rotation button (and that makes more sense
too). This was broken by a more or less recent Google commit moving the
floating rotation button handler to Launcher3 for taskbar. Manually
changing this to "menu_ime" in this overlay however gets 2-button nav
back on track.
Accessibility button, rotation suggestion button, IME switcher and menu
button (with extra downstream commit that returns support for this) now
work in 2-button again.

Thanks to SirRGB for bringing this issue to my attention.

Change-Id: I0a09ec0aa8b7dbfef02d1aaf0dedd2128156a6e7