releasetools: Fix an issue in image size computation.

When building a system image with system_root_image enabled, the size
computation should include files under both of in_dir (i.e. /system
files) and root (pointed by 'root_dir'). Because files from both
locations will end up into the built image. The files under root are
usually only a few MiBs, but should be accounted for especially in the
context of logical partitions (where the partition size will be
allocated based on the actual need). Note that we will still need some
"reserved space" (defined via BOARD_*_PARTITION_RESERVED_SIZE) to cover
the cost for filesystem and/or verity metadata.

This CL moves the combination of the two dirs up, before parsing and
computing other properties. This doesn't affect anything for a
successful image building path. It may however increase the time to
error out in certain error path, since it copies the files earlier now.

Test: python -m unittest test_build_image
Test: `make dist`
Test: Setup a target with PRODUCT_USE_LOGICAL_PARTITIONS == true and
      BOARD_SYSTEMIMAGE_PARTITION_RESERVED_SIZE == 20MiB. Build system
      image successfully.
Test: Setup a target with PRODUCT_USE_LOGICAL_PARTITIONS == true and
      BOARD_SYSTEMIMAGE_PARTITION_RESERVED_SIZE == 20MiB. Write a large
      file to root dir (PRODUCT_OUT_ROOT). The image building fails, but
      reporting a size that accounts for both of /system and root.
Change-Id: Idfb26b8e259626ba57ec3bd4f85d357c30e56163