android: Build native libraries also for x86

Requires an updated build script for Vstr.

src/frontends/android/jni/.gitignore

@@ -1,4 +1,4 @@
 openssl
 strongswan
-vstr/libvstr.a
 vstr/include
+vstr/*/libvstr.a

src/frontends/android/jni/Application.mk

@@ -0,0 +1,3 @@
+# select the ABI(s) to build for (see CPU-ARCH-ABIS.html in the NDK docs).
+# Vstr has to be built accordingly first.
+APP_ABI := armeabi x86

src/frontends/android/jni/vstr/Android.mk

@@ -2,7 +2,7 @@ LOCAL_PATH := $(call my-dir)
 
 include $(CLEAR_VARS)
 
-LOCAL_SRC_FILES := libvstr.a
+LOCAL_SRC_FILES := $(TARGET_ARCH)/libvstr.a
 
 LOCAL_MODULE := libvstr