Fix uses of KeyedVector

Constructor for AudioFlinger::mAudioHwDevs was missing, and so
AudioFlinger::findSuitableHwDev_l() could return an undefined pointer
if a non-0 module wasn't found.

A KeyedVector of Plain Old Data (POD) element type must specify the
default value in the constructor, or else the default will be undefined.

Minor:
 - Parameter had wrong type in constructor for AudioSystem::gOutputs.
 - Remove obsolete AudioSystem::gStreamOutputMap.

Change-Id: I9841493e018440e559d8b8b0e4e748ba2b2d365b