Always use OpenJDK9 for building

Remove support for compiling with javac from OpenJDK8.
We still target 1.8 by default, and OpenJDK8 prebuilts are still
required for the bootclasspath and running robolectric.

Bug: 38418220
Test: m java
Change-Id: I5686deb0ae4f9927192a039d08adc0117b2605dd