audio_hw: close mmap file descriptor to fix leak

The file descriptor for MMAP shared memory was created
and passed to AudioFlinger but never closed.
Now the FD is closed when the stream is goes to standby.

Bug: 134381208
Test: Get HAL pid.
Test:     adb shell ps | grep audio
Test:     adb shell ls -l /proc/{halpid}/fd | wc
Test: Run Oboetester or other app that uses MMAP
Test: Completely close the app.
Test:     adb shell ls -l /proc/{halpid}/fd | wc
Test: Count should NOT grow each time.
Change-Id: Ieaaf1c6bdc96e7ecf01cee23215fb39f79662111