Move adbd to an apex.

Test: adb shell "su 0 readlink /proc/\`pidof adbd\`/exe"
Change-Id: Ic71e78111a7201b1e5a8eb6b43a1ea689a655cd1
diff --git a/apex/com.android.adbd-file_contexts b/apex/com.android.adbd-file_contexts
new file mode 100644
index 0000000..3488de2
--- /dev/null
+++ b/apex/com.android.adbd-file_contexts
@@ -0,0 +1,2 @@
+(/.*)?                   u:object_r:system_file:s0
+/bin/adbd                u:object_r:adbd_exec:s0