Revert "Default to building with OpenJDK 9 -target 1.8."

After this revert CL topic, the default toolchain for Android
goes back to being OpenJDK 8.

This revert is being prepared ahead of time in case of
any problems with original change topic,
https://r.android.com/#/q/topic:bug_69449021

Bug: 69449021
Test: Treehugger

This reverts commit 0ae8b548.

Change-Id: Ief7646a94f1a264085cd299b4327d244b78a1537