default-permissions: Don't grant custom permission by default

* We can survive without granting this permission
  out of box, also fixes Exchange-less boot up
  exception.

D AndroidRuntime: Shutting down VM
E AndroidRuntime: *** FATAL EXCEPTION IN SYSTEM PROCESS: main
E AndroidRuntime: java.lang.IllegalArgumentException: Unknown permission: com.android.email.permission.READ_ATTACHMENT
E AndroidRuntime: 	at com.android.server.pm.permission.PermissionManagerService.grantRuntimePermission(PermissionManagerService.java:2103)
E AndroidRuntime: 	at com.android.server.pm.permission.PermissionManagerService.access$900(PermissionManagerService.java:121)
E AndroidRuntime: 	at com.android.server.pm.permission.PermissionManagerService$PermissionManagerServiceInternalImpl.grantRuntimePermission(PermissionManagerService.java:3032)
E AndroidRuntime: 	at com.android.server.pm.PackageManagerService.grantRuntimePermission(PackageManagerService.java:5800)
E AndroidRuntime: 	at android.app.ApplicationPackageManager.grantRuntimePermission(ApplicationPackageManager.java:684)
E AndroidRuntime: 	at com.android.server.pm.permission.DefaultPermissionGrantPolicy.grantRuntimePermissions(DefaultPermissionGrantPolicy.java:1217)
E AndroidRuntime: 	at com.android.server.pm.permission.DefaultPermissionGrantPolicy.grantDefaultPermissionExceptions(DefaultPermissionGrantPolicy.java:1381)
E AndroidRuntime: 	at com.android.server.pm.permission.DefaultPermissionGrantPolicy.grantDefaultPermissions(DefaultPermissionGrantPolicy.java:297)
E AndroidRuntime: 	at com.android.server.pm.PackageManagerService.systemReady(PackageManagerService.java:21731)
E AndroidRuntime: 	at com.android.server.SystemServer.startOtherServices(SystemServer.java:2037)
E AndroidRuntime: 	at com.android.server.SystemServer.run(SystemServer.java:529)
E AndroidRuntime: 	at com.android.server.SystemServer.main(SystemServer.java:356)
E AndroidRuntime: 	at java.lang.reflect.Method.invoke(Native Method)
E AndroidRuntime: 	at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:491)
E AndroidRuntime: 	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:908)

Change-Id: If2caad2b004820b10b7095a4bb4307d0f065c723