Monitor alsa device files to notify USB devices connection.

Only notify new USB device connection when there is any alsa device
file present. When the USB bus notifies new USB device connection, the
alsa device files may not be present. If new USB device connection is
notified immediately when the bus detects new device attached, the HAL
may not be able to open any input/output stream over USB devices given
the alsa device files may not be ready. In this CL, adding the monitor
to watch new alsa device files creation and only notify audio framework
about new USB device connection when there is nay alsa device file
ready.

Bug: 261538121
Test: play and record audio via USB headset
Change-Id: I2a93f4868f9db82c69dd839cef550244c7d9d3e7