Add BoardConfig flag TARGET_USES_OPENGLES_FOR_SCREEN_CAPTURE

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)

This is a better and safer approach than https://gerrit.omnirom.org/#/c/2670/.

Change-Id: I37437827434dbc3bec80956f1e199dbe8e3d255a