Preparation work for adb to support USB vendor Ids provided by SDK add-ons.

Added usb_vendors.* which handles creating (and deleting) a list of vendor ids.
This list is meant to be used everywhere the built-in lists (usb_osx), or the
built-in vendor IDs (transport_usb)  were used.

For now the list is only built with the built-in VENDOR_ID_*. Next step
is to read a small file created from all the SDK add-on.

Other misc changes: made is_adb_interface present only if ADB_HOST is true
to prevent accessing a list that doesn't exist (usb_vendors is only
compiled for the host version of adb).
diff --git a/adb.c b/adb.c
index 7a2b299..0b5ebac 100644
--- a/adb.c
+++ b/adb.c
@@ -29,6 +29,8 @@
 
 #if !ADB_HOST
 #include <private/android_filesystem_config.h>
+#else
+#include "usb_vendors.h"
 #endif
 
 
@@ -833,6 +835,7 @@
 
 #if ADB_HOST
     HOST = 1;
+    usb_vendors_init();
     usb_init();
     local_init();
 
@@ -916,6 +919,9 @@
     fdevent_loop();
 
     usb_cleanup();
+#if ADB_HOST
+    usb_vendors_cleanup();
+#endif
 
     return 0;
 }