[SPARK-14753][CORE] remove internal flag in Accumulable

## What changes were proposed in this pull request?

the `Accumulable.internal` flag is only used to avoid registering internal accumulators for 2 certain cases:

1. `TaskMetrics.createTempShuffleReadMetrics`: the accumulators in the temp shuffle read metrics should not be registered.
2. `TaskMetrics.fromAccumulatorUpdates`: the created task metrics is only used to post event, accumulators inside it should not be registered.

For 1, we can create a `TempShuffleReadMetrics` that don't create accumulators, just keep the data and merge it at last.
For 2, we can un-register these accumulators immediately.

TODO: remove `internal` flag in `AccumulableInfo` with followup PR

## How was this patch tested?

existing tests.

Author: Wenchen Fan <wenchen@databricks.com>

Closes #12525 from cloud-fan/acc.