Mark rkpd module SDKs as optional

This will generate SDK snapshots that use an rkpd specific Soong config
variable to allow a product to select whether to build from sources or
prebuilts.

Ignore-AOSP-First: rkpd is internal only
Bug: 266683341
Test: TARGET_BUILD_APPS=com.android.rkpd packages/modules/common/build/mainline_modules_sdks.sh --build-release=UpsideDownCake
      # Check the Android.bp file in out/dist/mainline-sdks/for-UpsideDownCake-build/current/com.android.rkpd/sdk/rkpd-sdk-current.zip
      # uses the rkpd_module/source_build Soong config variable and not
      # ANDROID/module_build_from_source.
Change-Id: Ib59b60eefef0d1e893e77aa93f911e80ecb696e3

build/mainline_modules_sdks.py

@@ -1006,6 +1006,8 @@ MAINLINE_MODULES = [
         apex="com.android.rkpd",
         sdks=["rkpd-sdk"],
         first_release=UpsideDownCake,
+        # Rkpd has always been and is still optional.
+        last_optional_release=LATEST,
     ),
     MainlineModule(
         apex="com.android.scheduling",