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