Clarified recommended use cases for NDK.

Added - "game engines" officially supported use case for NDK
Removed - "self-contained" ambiguous in general and confusing to someone who is trying to do game development in C++
Removed - "that don't allocate a lot of memory" game engines do allocate a lot of memory.

Change-Id: I098de65bf0c4b4ab4a85611426b68613692e52c5