Cleanup AudioSystem::getInputBufferSize error handling and caching

Previously, if the IAudioFlinger::getInputBufferSize failed,
it would return NO_ERROR but a zero buffer size value, which could
confuse the caller.  Now it returns BAD_VALUE in this case.

Also it would still cache the zero buffer size.  Now it does
not cache on failure.

Removed over-initialization of the cache globals.

Change-Id: I6835fcb56fe52535e018fc8c0c242115221b5d85