adb: move usb_* to client/usb_*.

Bug: http://b/31321337
Test: mma
Change-Id: I21eefab953737e87d211959616ac4f48926dd92c
diff --git a/adb/Android.mk b/adb/Android.mk
index 1ad77f1..41cb735 100644
--- a/adb/Android.mk
+++ b/adb/Android.mk
@@ -81,17 +81,17 @@
 
 LIBADB_darwin_SRC_FILES := \
     sysdeps_unix.cpp \
-    usb_osx.cpp \
+    client/usb_osx.cpp \
 
 LIBADB_linux_SRC_FILES := \
     sysdeps_unix.cpp \
-    usb_linux.cpp \
+    client/usb_linux.cpp \
 
 LIBADB_windows_SRC_FILES := \
     sysdeps_win32.cpp \
     sysdeps/win32/errno.cpp \
     sysdeps/win32/stat.cpp \
-    usb_windows.cpp \
+    client/usb_windows.cpp \
 
 LIBADB_TEST_windows_SRCS := \
     sysdeps/win32/errno_test.cpp \