should_flash_in_userspace signature change

We should change should_flash_in_userspace to work without
$ANDROID_PRODUCT_OUT being set and fall back on $OUT directory when
source isn't specified

This also fixes the flashing issue of flashing retrofit devices without
$ANDROID_PRODUCT_OUT set. If we
call fastboot update on a retrofit device, copy_avb_footer will resize
buf->sz of secondary partitions to the partition size, causing us to
send the unsparsed payload to the device -> "Invalid Size" issue.

This is because secondary partitions are considered physical partitions
on retrofit devices && should_flash_in_userspace wouldn't work if
$ANDROID_PRODUCT_OUT is set, so our first return clause in
copy_avb_footer doesn't catch this edge case causing us to resize the
buf-> sz incorrectly.

Did some testing, and also observed a case where flashing is able to
succeed despite $ANDROID_PRODUCT_OUT not being set before, so there
might still be some other edge case that we are still missing.

Test: fastboot update on sargo without $ANDROID_PRODUCT_OUT

Change-Id: I0e4781d96709b712f7d71657ec0d16d99b90214d