Fix EGLUtils::selectConfigForPixelFormat()

- renderscript now calls EGL directly instead of relying on this function
- surfaceflinger also does its own EGLConfig selection
- selectConfigForPixelFormat stays for legacy reason (many tests use it) but
it now only tries to match the alpha channel of the format rather than the
format itself.

this will allow implementations who don't support the exact formats
defined in the HAL to work properly.

Bug: 4998223

Change-Id: Ic664dfc14d5072a514b6f77a115d1521bfc1578f