Set TARGET_USES_OPENGLES_FOR_SCREEN_CAPTURE to fix screenshots

This flag forces usage flags:
GRALLOC_USAGE_HW_TEXTURE and GRALLOC_USAGE_HW_RENDERER
in captureScreenImplLocked regardless of useReadPixels value
This fixes the EGL_NO_IMAGE_KHR error returned from
eglCreateImageKHR (blank images returned from screenshot path)