Don't start adb if debugging is off

* Debug level None: adb is not supported
* Debug level app_only: adb is supported, but rooting is prohibited
* Debug level full: adb is supported and can be rooted

Bug: 201362865
Test: start a VM with `--debug none` and try to connect to the VM via
adb. It doesn't work as expected.

Change-Id: I95fe01e722d974ff1eb29469ac96abe7d785288d
diff --git a/microdroid/bootconfig.normal b/microdroid/bootconfig.normal
index 79e2b08..f7cdfc7 100644
--- a/microdroid/bootconfig.normal
+++ b/microdroid/bootconfig.normal
@@ -1 +1,8 @@
+# ro.debuggable is off
+androidboot.microdroid.debuggable=0
+
+# Console output is not redirect to the host-side.
 kernel.console = null
+
+# ADB is not enabled.
+androidboot.adb.enabled=0