Disable source javalib dexpreopt when using prebuilt apex

The apex_contributions in next builds use prebuilt apexes with source
java_sdk_library for javac. The dexpreopt rules of the source
java_sdk_library are disabled via
RELEASE_DEFAULT_MODULE_BUILD_FROM_SOURCE coupled with some complicated
special-casing in android/prebuilt.go

This special-casing breaks if we want to use prebuilts of some modules,
and sources of other modules. To enable per mainline module
toggelability, explicitly disable dexpreopt of source variant of
java_sdk_library if we use prebuilt of the containing apex.

Test: In internal, updated trunk_staging.scl's
RELEASE_APEX_CONTRIBUTIONS_ADSERVICES to use prebuilt followed by m
nothing

Change-Id: I32daefbd38338b396d6f07b899826b2869d8f8e1