Fix sbox in chdir mode

When running commands in chdir mode sbox needs to translate the
directory placeholders to paths relative to the sandbox directory
instead of relative to the top of the source tree.

Also translate relative PATH entries into absolute paths so they
are still valid when the current directory is changed.

Bug: 182612695
Test: manual
Change-Id: Idcbe20466888909d423d62788bc9c35f4e03b398