Avoid targeting Java 9 for targets with SDK version up to 29.

Targets with sdk_version values up to and including 29 should not use
Java language level 9, even if EXPERIMENTAL_JAVA_LANGUAGE_LEVEL_9=true
is set.

This change fixes a downstream issue.

Bug: 131678633
Test: EXPERIMENTAL_JAVA_LANGUAGE_LEVEL_9=true make droid tests docs cts java
Test: (same test in downstream branch)
Change-Id: Ic58e8df2e06d6e07bc42255314521831dc41b239