libs/ui: Move ScreenPartStatus into its own header

We need to remove all libs/ui dependencies on DisplayIdentification in
order to be able to move it back to SF/Display. ScreenPartStatus is one
such dependency.

Move ScreenPartStatus to its own header in libs/ui. Also replace
ScreenPartStatusToString() with ftl::enum_string.

Bug: 415078475
Flag: EXEMPT refactor
Test: presubmit
Change-Id: I82ba05d156cdf63fb497ba9b5110cb673e35cb68