[Android] SyncAdapter (API 5)

AccountManager: 管理手機裡的帳號

• The user enters credentials (username and password) once per account, granting applications access to online resources with "one-click" approval.
• Can generate auth tokens for applications, so the application doesn't need to handle passwords directly.

Authenticators: Validate accounts


若有實做 AbstractAccountAuthenticator 則能在 Account & Sync 新增同步帳號；

新增時，會觸發做以下設定的 service:

* Reference
- SampleSyncAdapter - Sample Sync Adapter
- class AccountManager
- class AbstractThreadedSyncAdapter
- class AccountAuthenticatorActivity
- class AbstractAccountAuthenticator