contrib/release_www.sh: put android versionCode in "version" file

This can be used by the F-Droid buildserver to detect new versions
and release updates automatically.

ref:
https://github.com/spesmilo/electrum/pull/9221#issuecomment-2386549086
https://f-droid.org/en/docs/Build_Metadata_Reference/#UpdateCheckMode

contrib/android/get_apk_versioncode.py

@@ -9,6 +9,7 @@ ARCH_DICT = {
     "arm64-v8a": "3",
     "armeabi-v7a": "2",
     "x86": "1",
+    "null": "0",
 }
 
 
@@ -58,4 +59,5 @@ if __name__ == '__main__':
         print(f"       should be one of: {list(ARCH_DICT.keys())}", file=sys.stderr)
         sys.exit(1)
     version_code = get_android_versioncode(arch_name=android_arch)
+    assert isinstance(version_code, int), f"{version_code=!r} must be an int."
     print(version_code, file=sys.stdout)