AudioPolicyManager: fix device enumeration

The AudioDeviceInfo only had PCM_FLOAT because the device enumeration was
stopping because of an error. The error was detected by new code
in the AudioFlinger.

The root error was not clearing a variable after a close.

Bug: 27457547
Change-Id: I529a26c8c0a688863bcf316ad365feddc303fe43
Signed-off-by: Phil Burk <philburk@google.com>