gui: Detect device resolution with TARGET_SCREEN_HEIGHT/WIDTH

 * Ease the compilation with Lineage/CM configurations
    when TW_THEME or DEVICE_RESOLUTION are not defined

Change-Id: I35a8ce651ba5333c8426baa3d70fcbc42a2e5e59
diff --git a/gui/Android.mk b/gui/Android.mk
index dab35a4..61bd102 100644
--- a/gui/Android.mk
+++ b/gui/Android.mk
@@ -98,6 +98,9 @@
 
 ifeq ($(TW_CUSTOM_THEME),)
     ifeq ($(TW_THEME),)
+        ifeq ($(DEVICE_RESOLUTION),)
+            DEVICE_RESOLUTION := $(TARGET_SCREEN_WIDTH)x$(TARGET_SCREEN_HEIGHT)
+        endif
         # This converts the old DEVICE_RESOLUTION flag to the new TW_THEME flag
         PORTRAIT_MDPI := 320x480 480x800 480x854 540x960
         PORTRAIT_HDPI := 720x1280 800x1280 1080x1920 1200x1920 1440x2560 1600x2560