envsetup: Really fix PROMPT_COMMAND handling

 * Add ANDROID_NO_PROMPT_COMMAND to not change it at all.
 * Export ANDROID_PROMPT_PREFIX so it can be used elsewhere.

Change-Id: Ib2a487404d3b8a367a21582e8a0ce69c2d212a9a
Merge remote branch 'cm/cm-10.1'