new Layout(...) calls Bangle.setUI(), causing uiRemove() to be called
whenever a new messages arrives, instead of only when the app exits.
This fixes that by unsetting Bangle.uiRemove before calling Layout, and
restoring it afterwards.
(And probably some other issues)
When we call Bangle.setUI to draw a new message, Bangle.uiRemove from the
previous message screen would save messages to flash and unset "special"
messages (like `music`).
Solution: delete Bangle.uiRemove before calling Bangle.setUI.