Add bit helper utility

- Add it to the base build (the first one in the hierarchy that includes
  the activity manager).
- Add tab completion commands to envsetup.sh

Test: . build/envsetup.sh ; make
Change-Id: Id24acaa0f17f198bda329016ff17170ad93624ec
diff --git a/envsetup.sh b/envsetup.sh
index e919891..7ea0a63 100644
--- a/envsetup.sh
+++ b/envsetup.sh
@@ -329,6 +329,8 @@
             . $f
         done
     fi
+
+    complete -C "bit --tab" bit
 }
 
 function choosetype()