sdk_version <= 28 implies Java 1.8

When sdk_version is equal to or less than 28, java_version is implied to
1.8 unless it is explicitly specified. It was 26 originally, and is now
updated to 28 which is the most recent API version.

Bug: 117069453
Test: EXPERIMENTAL_USE_OPENJDK9=true make apex_aidl_interface
Change-Id: I18ebe3ec51ada54103f6d203b9bfae8ec6ea5d90