android: Keep NDK API level at 19

The headers/libraries changed a lot with level 21 so that our app won't
run on devices with Android < 5 when built against it.  We currently
don't need any new native APIs so that should be fine.

src/frontends/android/jni/Application.mk

@@ -1,2 +1,3 @@
 # select the ABI(s) to build for (see CPU-ARCH-ABIS.html in the NDK docs).
 APP_ABI := armeabi x86 mips
+APP_PLATFORM := android-19