soundpool: reuse channel for same sample if available

Reuse channel for same sample if the channel completed
current playback and is not reallocated to another sample,
i.e. not stolen by other sample.

authored-by: Ashish Jain <ashishj@codeaurora.org>

Change-Id: Ibe7ee318c7dc11f3c4fd3a2f57d861318b10973b
Signed-off-by: Glenn Kasten <gkasten@google.com>