Allow the qemu.sf.lcd_density property to override the value of ro.sf.lcd_density

ro.sf.lcd_density is usually defined in the build.prop file which is parsed by init
before anything else. Since its name begins with "ro.", this property is write-once
and cannot later be modified, e.g. in /system/etc/init.goldfish.sh.

In other words, you cannot use "emulator -prop ro.sf.lcd_density=<value>", since
it is impossible to override the value defined in build.prop

This patch modifies the system to recognize "qemu.sf.lcd_density" as an override
value, which can be set with "emulator -prop qemu.sf.lcd_density=<value>", forcing
a specific density.

A later patch will allow the emulator to automatically set this property depending
on AVD hardware configuration settings.

core/java/android/util/DisplayMetrics.java

@@ -37,8 +37,7 @@ public class DisplayMetrics {
      * The device's density.
      * @hide
      */
-    public static final int DEVICE_DENSITY = SystemProperties.getInt("ro.sf.lcd_density",
-            DEFAULT_DENSITY);
+    public static final int DEVICE_DENSITY = getDeviceDensity();
 
     /**
      * The absolute width of the display in pixels.
@@ -161,4 +160,13 @@ public class DisplayMetrics {
             ", height=" + heightPixels + ", scaledDensity=" + scaledDensity +
             ", xdpi=" + xdpi + ", ydpi=" + ydpi + "}";
     }
+
+    private static int getDeviceDensity() {
+        // qemu.sf.lcd_density can be used to override ro.sf.lcd_density
+        // when running in the emulator, allowing for dynamic configurations.
+        // The reason for this is that ro.sf.lcd_density is write-once and is
+        // set by the init process when it parses build.prop before anything else.
+        return SystemProperties.getInt("qemu.sf.lcd_density",
+                SystemProperties.getInt("ro.sf.lcd_density", DEFAULT_DENSITY));
+    }
 }

libs/surfaceflinger/DisplayHardware/DisplayHardware.cpp

@@ -193,6 +193,14 @@ void DisplayHardware::init(uint32_t dpy)
         LOGW("ro.sf.lcd_density not defined, using 160 dpi by default.");
         strcpy(property, "160");
     }
+
+    /* Override the property value if qemu.sf.lcd_density is defined. */
+    {
+        char  qemu_property[PROPERTY_VALUE_MAX];
+        if (property_get("qemu.sf.lcd_density", qemu_property, NULL) > 0) {
+            strlcpy(property, qemu_property, sizeof property);
+        }
+    }
     mDensity = atoi(property) * (1.0f/160.0f);