Protect LOGIN_ACCOUNTS_CHANGED receiver in SyncManager

BUG: 17359770
Reuse the Manifest.permission.ACCOUNT_MANAGER to protect this receiver,
because when this intent lands it kicks off a sync for every account/provider
on the device.
Semantically this means "if you can't talk to an authenticator to do things like
get a token or add an account, then you can't send an intent to the SyncManager
that the accounts on the device have changed in some drastic way"

Change-Id: I7eccc38e135cb00ebb0aafc69afc802fe1c4f219