[SPARK-17648][CORE] TaskScheduler really needs offers to be an IndexedSeq

## What changes were proposed in this pull request?

The Seq[WorkerOffer] is accessed by index, so it really should be an
IndexedSeq, otherwise an O(n) operation becomes O(n^2).  In practice
this hasn't been an issue b/c where these offers are generated, the call
to `.toSeq` just happens to create an IndexedSeq anyway.I got bitten by
this in performance tests I was doing, and its better for the types to be
more precise so eg. a change in Scala doesn't destroy performance.

## How was this patch tested?

Unit tests via jenkins.

Author: Imran Rashid <irashid@cloudera.com>

Closes #15221 from squito/SPARK-17648.