Add the 'leftovers' command.

'lunch' now saves its arguments to $ANDROID_BUILD_TOP/.leftovers. 'leftovers' reads this file and invokes lunch with those arguments.

When USE_LEFTOVERS=1, 'leftovers' is run when envsetup.sh is invoked.

Test: Tested all permutations of running lunch and leftovers
Change-Id: I8ef1de0d2034e115bac110658172c9197416ff50