Make sure unmatched autenticator/account-types doesn't crash Contacts.

If the Contacts-app didn't find a valid authenticator for a specific
account-type, it would previously throw an exception and crash.
Now the account-type is silently ignored.

Installing an app with unmatching authenticator/accounts like, e.g:

<account-authenticator
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:accountType="com.abc.def"
    android:icon="@drawable/icon"
    android:smallIcon="@drawable/icon"
    android:label="@string/label"
/>

<sync-adapter
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:contentAuthority="com.android.contacts"
    android:accountType="com.abc.ghi"
    android:supportsUploading="true"
/>

would otherwise crash Contacts.

Change-Id: Idefa3266e1624246de97d1eae6612747b52f8c60