[GraphicsEnvironment] Avoid null access.

Currently when debug layer apk is not installed, GraphicsEnvironment
logs the message and proceeds, which results in null pointer access
crash when later trying to access primaryAbi. Avoid null access by an
early return statement.

Bug: N/A
Test: specify not installed debug layer apk and no crash
Change-Id: I4f3b86b0a8cfff4fc608b3a59ebf0872db0465ff