Add flag to control if to target java 21 by default

Bug: 342332820
Change-Id: I76a3212ca6ee00ef9f78839d5873b49aff4f75af
Test: m
Test: lunch aosp_cf_x86_64_phone-trunk_staging-eng && \
       build-flag set RELEASE_TARGET_JAVA_21 true && m
1 file changed