[android] Restore prior GLContext if exists

Apparently some versions of Android, notable 4.4.4 running
on my Nexus 5, creates a GLContext on the Android UIThread
which is the one we are now using for rendering after #2909.

If the context is not restored, nothing gets rendered (or
sometimes partially, or artifacts) because Android will try
to do GL stuff on the context used by Mapbox GL and mess things up.

platform/android/src/native_map_view.cpp

@@ -174,6 +174,11 @@ void NativeMapView::invalidate() {
 }
 
 void NativeMapView::render() {
+    EGLDisplay currentDisplay = eglGetCurrentDisplay();
+    EGLSurface currentReadSurface = eglGetCurrentSurface(EGL_READ);
+    EGLSurface currentDrawSurface = eglGetCurrentSurface(EGL_DRAW);
+    EGLContext currentContext = eglGetCurrentContext();
+
     activate();
 
     if(sizeChanged){
@@ -195,7 +200,19 @@ void NativeMapView::render() {
         mbgl::Log::Info(mbgl::Event::Android, "Not swapping as we are not ready");
     }
 
-    deactivate();
+    if (currentContext != context && currentContext != EGL_NO_CONTEXT) {
+        if (currentDisplay == EGL_NO_DISPLAY) {
+            currentDisplay = display;
+        }
+
+        if (!eglMakeCurrent(currentDisplay, currentDrawSurface, currentReadSurface, currentContext)) {
+            mbgl::Log::Error(mbgl::Event::OpenGL, "eglMakeCurrent() returned error %d",
+                             eglGetError());
+            throw std::runtime_error("eglMakeCurrent() failed");
+        }
+    } else {
+        deactivate();
+    }
 }
 
 mbgl::Map &NativeMapView::getMap() { return *map; }