Add var to enable/disable tethering next APEX

This is necessary to disable the .inprocess APEX, in branches that only
use stable APIs, as it depends on unstable stubs. override_apex does not
support defaults.

Bug: 190249673
Test: m
Change-Id: Ib4a27b65f3c5b6d4b0ac69e85cf671038efc5744
diff --git a/Tethering/apex/Android.bp b/Tethering/apex/Android.bp
index c72d3a6..afbcd76 100644
--- a/Tethering/apex/Android.bp
+++ b/Tethering/apex/Android.bp
@@ -31,6 +31,7 @@
     // package names and keys, so that apex will be unused anyway.
     apps: ["TetheringNext"], // Replace to "Tethering" if ConnectivityNextEnableDefaults is false.
 }
+enable_tethering_next_apex = true
 // This is a placeholder comment to avoid merge conflicts
 // as the above target may have different "enabled" values
 // depending on the branch
@@ -133,6 +134,7 @@
     name: "com.android.tethering.inprocess",
     base: "com.android.tethering",
     package_name: "com.android.tethering.inprocess",
+    enabled: enable_tethering_next_apex,
     apps: [
         "ServiceConnectivityResources",
         "InProcessTethering",