emulator: opengl: build gralloc driver by default

This patch forces the default definition of BUILD_EMULATOR_OPENGL_DRIVER
to true by default, which is the common case where we will build the
goldfish-specific gralloc hardware module to allow SurfaceFlinger to
use GLES emulation exclusively.

The only reason to define thi to 'false' today is to debug specific
apps, but this requires displaying the GL output in a different window
than the emulator to be usable.

Change-Id: Iaf03a9c780449bb6b991928d324a088021a73203
1 file changed