reimplement wifi display hack with new external display SF framework

fix a few bugs with external displays

- HWComposer doesn't really handle multiple displays yet
  so there is a lot of ugliness there

- We also need to make sure that external displays are not
  blanked by default

- due to some EGL limitations surfaces being swapped need
  to be current

Change-Id: I82bff05b43bcebd8da863c7c76b4edbc3bc223a9
diff --git a/services/surfaceflinger/DisplayDevice.cpp b/services/surfaceflinger/DisplayDevice.cpp
index 4cae692..a379111 100644
--- a/services/surfaceflinger/DisplayDevice.cpp
+++ b/services/surfaceflinger/DisplayDevice.cpp
@@ -140,6 +140,9 @@
     mFormat  = format;
     mPageFlipCount = 0;
 
+    // external displays are always considered enabled
+    mScreenAcquired = mId >= DisplayDevice::DISPLAY_ID_COUNT;
+
     // initialize the display orientation transform.
     DisplayDevice::setOrientation(DisplayState::eOrientationDefault);
 }