Fix readImmediately for CTS

The readImmediately API (only used by CTS so far) was broken by a
previous change that unknowingly changed the semantics of ready().
Fixing this so that CTS tests work again.

Change-Id: I6970713eae2a01dd07e30778ca8ee0d9bc7dfeb9
Fixes: 111125833
Test: CTS