EglMacApi: Make sure that some returned EGL configs have alpha == 0

This is what the pixel format attribute lists in
MacPixelFormatsAttribs.m try to achieve, but despite this, alpha
is nonzero in every returned configuration on certain (all?)
machines (at least on 10.8.5 on a nvidia gpu). This means that
EGL won't return any configs at all with alpha == 0.

The default config chooser in GLSurfaceView requires a config with
alpha == 0. This means that previously, this view failed to start up
on the emulator on OS X, unless set up with a non-default config
chooser.

Change-Id: I2bf3e92a026c525a97d6746e491d920ce127787f