android: with_jni silently ships no odr-core-java.jar when no JDK is on the build machine

[andiwand]

On android, jni/CMakeLists.txt looks for a JDK without REQUIRED since #629:

if (ANDROID)
    find_package(Java 11 COMPONENTS Development)
else ()
    find_package(Java 11 REQUIRED COMPONENTS Development)
    find_package(JNI REQUIRED)
endif ()
...
install(TARGETS odr_jni LIBRARY DESTINATION "${CMAKE_INSTALL_LIBDIR}" COMPONENT jni)

if (NOT Java_FOUND)
    return()
endif ()

So a build machine with no javac on PATH and no JAVA_HOME configures, compiles
and installs happily, and produces a package with libodr_jni.so and no
odr-core-java.jar — even though the option that was asked for is with_jni=True,
and the comment right above says the android odrcore package "ships
odr-core-java.jar next to libodr_jni.so and OpenDocument.droid takes both out of
it".

It bit me bumping OpenDocument.droid to 6.1.0. The conan build succeeded, and the
failure surfaced much later and somewhere else:

FileNotFoundError: [Errno 2] No such file or directory:
'~/.conan2/p/b/odrcod31d961a414ad/p/share/java/odr-core-java.jar'
ERROR: [Errno 2] No such file or directory: ...
Execution failed for task ':app:conanInstall-armv8'

which reads like a broken deployer rather than "your build environment has no JDK,
so half the package is missing". Rebuilding with JAVA_HOME exported fixed it. On
6.0.1 the same environment failed at configure time with a clear
Could NOT find Java, which was the better outcome.

The reason to relax it doesn't seem to cover the jar. What android cannot do is
find_package(JNI) — the NDK sysroot ships jni.h and the symbols come from the
runtime — and the AAR build doesn't need the jar because android/build_native.py
asks for --target odr_jni only, so it never reaches add_jar anyway. Java itself
is as findable on an android build as on any other.

Two ways out, either is fine by me:

• keep find_package(Java 11 REQUIRED COMPONENTS Development) unconditional and only
  put find_package(JNI REQUIRED) behind if (NOT ANDROID). android/build_native.py
  runs under gradle, which has a JDK by definition.
• or make it explicit with an option, e.g. ODR_JNI_JAR defaulting to ON, REQUIRED
  when it is on, and set to OFF by the AAR build. Then "no jar" is something a caller
  asked for rather than something the environment decided.

Happy to send a PR for whichever you prefer.