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
envsetup: More PROMPT_COMMAND

* Get rid of ANDROID_NO_PROMPT_COMMAND, the pre-existing STAY_OFF_MY_LAWN
does the same thing
* Remove any pre-existing android prefixes to avoid accumulation
scenarios in consecutive builds (i.e., multiple build identification
strings in hardstatus)

Change-Id: I86661e066e18ea6ad0c742fa1dc6555780fa5441