Add an adb_tradeinmode type for restricted adbd.

This adds sepolicy for a super restricted adbd mode. Currently, this
mode has just enough permissions to handle adb connection.

It also adds a new property, persist.adb.tradeinmode, which can be used
to enter this restricted version of adbd.

Test: manual test
Bug: 307713521
Change-Id: I99963f27ebab615332cb971701d1c06ea01332a1
diff --git a/private/attributes b/private/attributes
index fe50b0d..2d6181d 100644
--- a/private/attributes
+++ b/private/attributes
@@ -15,3 +15,5 @@
 attribute sdk_sandbox_all;
 # The SDK sandbox domains for the current SDK level.
 attribute sdk_sandbox_current;
+# Common to adbd and adbd_tradeinmode.
+attribute adbd_common;