[build] Cursory JDK 9 support

[jonpryor]

We have a VSTS+macOS build machine which has JDK 9 as the default JDK,
thus causing all xamarin-android builds to fail, as gradlew and
JDK 9 don't directly mix:

Executing: ./gradlew assembleDebug --stacktrace
...
org.gradle.api.ProjectConfigurationException: A problem occurred configuring project ':library'.
  ...
Caused by: org.gradle.internal.event.ListenerNotificationException:
Failed to notify project evaluation listener.
  ...
Caused by: java.lang.NoClassDefFoundError: javax/xml/bind/annotation/XmlSchema
  ...
Caused by: java.lang.ClassNotFoundException: javax.xml.bind.annotation.XmlSchema

When using JDK 9, gradlew fails because something within it
attempts to use the deprecated type
javax.xml.bind.annotation.XmlSchema, and JDK 9 doesn't provide
deprecated modules in the default $CLASSPATH.

Knowing that the VSTS machine also has JDK 8 installed, we've tried
to "filter out" JDK 9 so that it wouldn't be used; see e.g. a3c4358.
Unfortunately that doesn't work, because on macOS the default JDK used
is always the JDK with the highest version number, and
this can't be easily changed system-wide because changing this
behavior requires "removing" a Info.plist file to prevent the JDK
from being "seen" by /usr/bin/java:

# Don't use JDK 9 by default on macOS:
$ cd /Library/Java/JavaVirtualMachines/jdk-9.0.4.jdk/Contents
$ mv Info.plist Info.plist.disabled

(Renaming Info.plist.disabled to Info.plist will restore JDK 9 as
the default JDK on the machine.)

A "proper" fix would presumably involve using and exporting
$JAVA_HOME everywhere, which isn't currently the case.
This is something that should be explored.

In the meantime, a question: Can we build with JDK 9?
Is it even possible in the first place?

Turns out, yes, it is possible. The above gradlew error can be
fixed by providing the (new-in-JDK9) --add-modules option, so that
the java.xml.bind module is accessible:

$ ANDROID_HOME=... JAVA_OPTS="--add-modules java.xml.bind" ./gradlew assembleDebug --stacktrace --no-daemon
# ...works!

Unfortunately, other parts xamarin-android get in the way, e.g. the
<ResolveSdks/> task doesn't correctly parse the version number out
of java -version, and JDK 9 javac requires -source and -target
when -bootclasspath is used.

Update the build system and tests so that things can be built under
JDK 9.

Note: This does NOT mean that building and/or using JDK 9
will be commercially supported in any way. (It might not even work!)

[dellis1972]

Any way we can merge these Xamarin.Android.BindingResolveImportLib*.targets files into one? For the most part the BuildJar and CleanJar targets are identical.

[jonpryor]

Done. There is now a new build-tools/scripts/Jar.targets file which contains a BuildTestJarFile target. This takes the @(TestJarEntry) item group -- which is assumed to contain Java source code -- compiles it with javac, and then jars it into %(TestJarEntry.OutputFile). Through the joy of Target batching, this allows a project to fairly easily generate any number of output .jar files without too much difficulty.

Merging has been completed.