Build updatable-bcp-packages.txt from prebuilts in sdk snapshot

Previously, the java_import and java_sdk_library_import modules did not
make their permitted_packages available to the rule that generates the
updatable-bcp-packages.txt file. This change corrects that.

Bug: 193763688
Test: m nothing
      - Added unit tests, which all failed and then fixed the tests.
Change-Id: If0706e4551a331b48d383123088e63924dded48b

java/sdk_library.go

@@ -1999,6 +1999,12 @@ func sdkLibraryImportFactory() android.Module {
 	return module
 }
 
+var _ PermittedPackagesForUpdatableBootJars = (*SdkLibraryImport)(nil)
+
+func (module *SdkLibraryImport) PermittedPackagesForUpdatableBootJars() []string {
+	return module.properties.Permitted_packages
+}
+
 func (module *SdkLibraryImport) Prebuilt() *android.Prebuilt {
 	return &module.prebuilt
 }