envsetup: add quotes around PROMPT_COMMAND

Better compatibility for zsh.

Change-Id: Ie010c30580e67e3b5357e3cc869114a525762677
Merge "envsetup: add quotes around PROMPT_COMMAND" into cm-10.1
Merge commit '7da16d025b2c972200ffb5b308fe1683e62e564a' into cm-10.1