Allow missing java_sdk_library files in AllowMissingDependencies builds

java_sdk_library modules in builds with AllowMissingDependencies may
be missing defaults modules that cause them to look for api files that
they normally wouldn't need.  Move the error to runtime so it doesn't
block the build unless the branch tries to build that module.

Test: prebuilts/build-tools/build-prebuilts.sh in aosp-build-tools
Change-Id: I279b0cd8493779f972c0ac02235967c10b35a5a0