Context:
https://stackoverflow.com/questions/47627499/does-android-studio-3-support-java-9-for-android-development

At the current time, we will not be able to use JDK 9 for Android. Some
of our build agents (VSTS), now have JDK 9 installed, so we need to
make a few changes to make sure JDK 8 is picked up instead.

Changes:
- Create a `JI_MAX_JDK` option, as a way for `xamarin-android` to
exclude JDK 9
- Create new `JI_JAVAC_PATH` and `JI_JAR_PATH` make variables, which
will be the full path to `javac` and `jar`
- Use `awk` to filter on <= `JI_MAX_JDK`
- Use `sed` to find the JDK version number, see options of folder names
below
- `sort -n` should be used to sort numerically
- Set `JI_JAVAC_PATH` and `JI_JAR_PATH` to their full paths
- Support both `Darwin` and `Linux`, Windows support is handled in
`xamarin-android` currently

~~Known JDK folder names~~

macOS:
```
1.6.0.jdk
jdk1.7.0_79.jdk
jdk1.8.0_101.jdk
jdk1.8.0_152.jdk
jdk-9.0.1.jdk
```

Ubuntu:
```
java-8-openjdk-amd64
java-9-openjdk-amd64
```