libprocessgroup: Fix file mode parsing that requires octal values

JSON supports only decimal values and therefore file mode which should
use octal values has to be represented as strings. Fix the implicit
octal-to-decimal conversion and convert from string to octal in the code.

Bug: 111307099
Test: confirmed by verifying correct file permissions

Change-Id: I3ef9de0aea259f93bf74efeffca72d37d4740e15
Signed-off-by: Suren Baghdasaryan <surenb@google.com>