envsetup: Fix $PROMPT_COMMAND

Change I200902f135d0c5c620a8eb3ce9cefdba318cf9d3 broke the android
PROMPT_COMMAND on scenarios where the existing prompt already had
a hardstatus adjustment, since appending it to Android's just clobbered
Android's in favor of the original.

So instead of appending them, inject android's at the beggining of the
existing one (and create a default if none exists)

Change-Id: I1fbcbfdb6220d886dda3662da498d1759a4c195b
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