Add ftl support to RenderEngine enums

Omitting the `k` enum prefix technically goes against Android's general
C++ guidelines, but allows ftl enum calls to be chained together while
remaining legible, e.g. "GraphiteVk" instead of "kGraphitekVk".
There appears to already be precedent where ftl enums are used.

An alternative to ftl (and the associated renaming of enum values) could
be to manually create helper toString functions, but that seems
unnecessary.

Bug: N/A
Test: compiles
Flag: EXEMPT refactor
Change-Id: I9c1e57882587eba3f88eab0f91248c2d79b2d145