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.full_debuggable b/microdroid/bootconfig.full_debuggable
index d67cd76..f6afdcf 100644
--- a/microdroid/bootconfig.full_debuggable
+++ b/microdroid/bootconfig.full_debuggable
@@ -1 +1,10 @@
+# ro.debuggable is set.
+androidboot.microdroid.debuggable=1
+
+# Kernel message is exported.
 kernel.printk.devkmsg=on
+
+# ADB is supported and rooting is possible. Note that
+# ro.adb.secure is still 0 (see build.prop) which means that adbd is started
+# unrooted by default. To root, developer should explicitly execute `adb root`.
+androidboot.adb.enabled=1