Set --noautodetect_server_javabase in mixed build bazel.

If a valid JAVA_HOME is set in the environment when running
bazel with --experimental_repository_disable_download, bazel
will unsuccessfully attempt to download rules_java. The above
flag has the practical effect of preventing the download attempt
(some clues as to why in the doc at go/aosp-bazel-jdk).

Note that this also prevents building java targets in mixed mode.
Eventually, we will pull in the java_rules into AOSP so they don't
need to be downloaded.

Test: build/bazel/ci/mixed_libc.sh
Change-Id: Icdb186c18a3334f4caa17243a8620a1cc8c1db0e