Update jni build script to java jdk 8+.

With jdk8, `javac` has an option `-h` to generate the header files of
native classes.
So there is no need to run `javah` several times.

As there is now only one command to run (`javac`), there is no need for
the wrapper script `gen_kiwix.sh`.

Fix #167

src/android/gen_kiwix.sh

@@ -1,16 +0,0 @@
-#!/usr/bin/env bash
-
-set -e
-
-BUILD_PATH=$(pwd)
-
-echo "javac -d $BUILD_PATH/src/android $@"
-javac -d $BUILD_PATH/src/android/ "$@"
-
-
-cd $BUILD_PATH/src/android
-echo "javah -jni org.kiwix.kiwixlib"
-javah -jni org.kiwix.kiwixlib.JNIKiwix
-javah -jni org.kiwix.kiwixlib.JNIKiwixReader
-javah -jni org.kiwix.kiwixlib.JNIKiwixSearcher
-cd $BUILD_PATH

src/android/meson.build

@@ -1,6 +1,4 @@
 
-jni_generator = find_program('gen_kiwix.sh')
-
 kiwix_jni = custom_target('jni',
   input: ['org/kiwix/kiwixlib/JNIKiwix.java',
           'org/kiwix/kiwixlib/JNIKiwixReader.java',
@@ -14,7 +12,7 @@ kiwix_jni = custom_target('jni',
            'org_kiwix_kiwixlib_JNIKiwixReader.h',
            'org_kiwix_kiwixlib_JNIKiwixSearcher.h',
            'org_kiwix_kiwixlib_JNIKiwixSearcher_Result.h'],
-  command:[jni_generator, '@INPUT@']
+  command:['javac', '-d', '@OUTDIR@', '-h', '@OUTDIR@', '@INPUT@']
 )
 
 kiwix_sources += [