Building mainline modules from source is now the default

RELEASE_APEX_CONTRIBUTIONS_* flags per module will be used to toggle
between sources and prebuilts of a mainline module in ap3a and above in
internal and partner builds.

Test: git_aosp-main-with-phones:aosp_cf_arm64_only_phone-next-userdebug
https://android-build.corp.google.com/builds/abtd/run/L61500030003577619

Test: lunch gsi_partner_arm-ap31-userdebug in v-aml-prebuilt-dev
Test: m out/soong/prebuil_info.json # identical before and after

Bug: 308188056
Change-Id: I308f071763297eb95ca5c8fb0191c8bb794789f7