USB MIDI: Fix incorrect timestamps

Timestamp is calculated before waiting for USB data. When USB data comes in,
the timestamp from when it starts to wait is used. This makes the timestamp
very inaccurate.

The fix is to move "timestamp = System.nanoTime()" after getting the USB
response.

Bug: 267519645
Test: Connect phone with MIDI Scope app to USB MIDI 2.0 keyboard
Test: Play notes on the keyboard and verify timestamps look reasonable
Change-Id: Ic3bccd89b65787badf3139964983572f83d99eaa