Fix adb function.

It was broken by the last change (because unlike `which adb`, `command
adb` does not search $PATH if there's already an entry in the hash).
No-one's noticed in almost a year, but this has caused (rare) confusion
in the past.

Bug: http://b/24473359
Change-Id: I6bbb207992449132ba8563a8bcf298256c599c0d

envsetup.sh

@@ -1085,7 +1085,12 @@ function cproj()
 }
 
 function adb() {
-   command adb "${@}"
+    local ADB=$(which adb)
+    if [ -z "$ADB" ]; then
+        echo "Command adb not found; try lunch (and building) first?"
+        return 1
+    fi
+    $ADB "${@}"
 }
 
 # simplified version of ps; output in the form