envsetup: Don't tread on my $PROMPT_COMMAND

 * I want my own PROMPT_COMMAND to coexist with Android's, so stash it
   in the environment and add it to the new command.

Change-Id: I200902f135d0c5c620a8eb3ce9cefdba318cf9d3

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

envsetup: add quotes around PROMPT_COMMAND

Better compatibility for zsh.

Change-Id: Ie010c30580e67e3b5357e3cc869114a525762677