libui: Remove STL from Fence

Renames Fence::hasSignaled to Fence::getStatus, and changes its return
type from std::experimental::optional (STL is no longer permitted in
libui) to an enum class. Also updates its one caller.

Test: Builds and sailfish boots
Change-Id: I682039b9fd88bf79f1e2a955944c4a564ed6f60b