Don't depend on classes-header.jar when turbine is disabled

TARGET_BUILD_APPS sets TURBINE_ENABLED := false, which triggers
an issue where a dependency is added on classes-header.jar but
there is no rule to create it.  Skip creating the dependency
when TURBINE_ENABLED is false for now.

Bug: 123183568
Test: m TARGET_PRODUCT=mainline_modules_arm UNBUNDLED_BUILD_SDKS_FROM_SOURCE=true TARGET_BUILD_APPS=com.android.media
Change-Id: I8ec3e244b8f9618c58f4ac57329a5a07f01dc797