Add a new option OVERRIDE_JLINK_VERSION_NUMBER.

This is a counterpart to the existing OVERRIDE_ANDROID_JAVA_HOME,
which is not very useful by itself in a Java module world. All jlink
commands will fail if a user sets OVERRIDE_ANDROID_JAVA_HOME without
setting OVERRIDE_JLINK_VERSION_NUMBER to the corresponding value
(unless they happen to be specifying a different OpenJDK 11
toolchain).

Test: OVERRIDE_ANDROID_JAVA_HOME=my-jdk10 OVERRIDE_JLINK_VERSION_NUMBER=10 m core-all-system-modules (with an OpenJDK 10 toolchain symlinked at my-jdk10 just for testing)
Test: zcat out/verbose.log.gz | grep 'jmod create'
Bug: 131683177
Change-Id: I31542e7f46303bba538d998171a3efe6d0215c7b