Move adbd's legacy USB implementation to fastboot.

This code path is effectively dead in adbd, and fastboot's dependency on
libadbd makes it hard to refactor adbd's dependencies.

Bug: http://b/150317254
Test: built and flashed aosp_walleye-eng
Change-Id: I5118136d32fdcbbd011559ed0a4a71e1dc7bf064
diff --git a/fastboot/Android.bp b/fastboot/Android.bp
index a757d56..884856d 100644
--- a/fastboot/Android.bp
+++ b/fastboot/Android.bp
@@ -115,6 +115,7 @@
         "device/fastboot_device.cpp",
         "device/flashing.cpp",
         "device/main.cpp",
+        "device/usb.cpp",
         "device/usb_client.cpp",
         "device/utility.cpp",
         "device/variables.cpp",
@@ -125,7 +126,6 @@
         "android.hardware.boot@1.1",
         "android.hardware.fastboot@1.0",
         "android.hardware.health@2.0",
-        "libadbd",
         "libasyncio",
         "libbase",
         "libbootloader_message",