adbd: add usb thread spawn logging.

Bug: http://b/141959374
Test: adbd shell killall adbd; adb wait-for-device logcat | grep UsbFfs
Change-Id: Id5bbfd6d2198005bf10b94c691499059e130afe7
(cherry picked from commit e3d34e1f8bd6c7df8a252f5ce734acf872383f56)
1 file changed