Enable adbd on microdroid

The following system properties are forcibly set to make adbd function.

ro.apex.updatable=true : otherwise, apexd doesn't activate critical
APEXes like the runtime APEX

ro.adb.secure=0 : to bypass the adb authentication

ro.debuggable=1 : to be able to use ro.adb.secure=0

This change also updates README.md file for the instruction to use adb.

Bug: 181728474
Test: adb shell works towards microdroid. See microdroid/README.md

Change-Id: Ica405e8bd4d2d1ef03e545aef5ed07ddc4bea34c
diff --git a/microdroid/init.rc b/microdroid/init.rc
index 9500f7c..2c32b28 100644
--- a/microdroid/init.rc
+++ b/microdroid/init.rc
@@ -13,6 +13,9 @@
 # Cgroups are mounted right before early-init using list from /etc/cgroups.json
 on early-init
     start ueventd
+    setprop ro.apex.updatable true
+    setprop ro.debuggable 1
+    setprop ro.adb.secure 0
 
     # Generate ld.config.txt
     exec -- /system/bin/bootstrap/linkerconfig --target /linkerconfig