Fix build: platform_apis, missing AIDL import, private-scope leak

- Android.bp: sdk_version is empty (platform APIs) but platform_apis
  wasn't set, which Soong doesn't just reject cleanly -- it panics.
- PawletCacheService.kt was missing the pawletos.cache.IPawletCacheService
  import (different package than the file's own os.pawlet.cache).
- CacheToken.isExpired called CacheTokenVerifier's private parseIso8601,
  invisible outside that object; moved it to file scope so both can use it.

Android.bp

@@ -23,6 +23,7 @@ android_app {
     // No hidden-API/privileged-permission surface needed — just platform-signed
     // so BgUpd (also platform-signed) can hold the signature-level BIND_RESOLVER
     // permission this app defines.
+    platform_apis: true,
     certificate: "platform",
     system_ext_specific: true,