New AVD property to control presence of hw main keys.

This allows injecting a new runtime property into the system
running on the emulator to control whether to display the new
software back/home keys.

Change-Id: I2bf7cc5c291752090187463332219638e883d052
diff --git a/vl-android.c b/vl-android.c
index a0041dc..322ddca 100644
--- a/vl-android.c
+++ b/vl-android.c
@@ -3689,6 +3689,9 @@
         hwLcd_setBootProperty(density);
     }
 
+    /* Initialize presence of hardware nav button */
+    boot_property_add("qemu.hw.mainkeys", android_hw->hw_mainKeys ? "1" : "0");
+
     /* Initialize TCP dump */
     if (android_op_tcpdump) {
         if (qemu_tcpdump_start(android_op_tcpdump) < 0) {