Bump Tethering apex version code for S

The version code in the manifest is only used for development
builds, as release builds dynamically replace it.

319999900 is appropriate as it indicates that the module is compatible
with S and earlier, and allows installing development versions on any
S build, which use 31xxxxxxx versions. It will also still allow installing
production T modules (32xxxxxxx) on a development S build. This is
convenient for local testing.

The same convention is also used for other modules including
NetworkStack.

Bug: 193853555
Test: m com.google.android.tethering, check version code

Change-Id: Ib8905a641780bed490a38327facee4a76e7770a3

Tethering/apex/manifest.json

@@ -1,4 +1,4 @@
 {
   "name": "com.android.tethering",
-  "version": 309999900
+  "version": 319999900
 }