Finish fixing Zygote descriptor leakage problem

In order to prevent Zygote descriptors from leaking into the child
environment, they should be closed by the forked-off child process
before the child switches to the application UID.  These changes close
the descriptors via dup2(), substituting a descriptor open to
/dev/null in their place; this allows the Zygote Java code to close
the FileDescriptor objects cleanly.

This is a multi-project change:	dalvik, art, libcore, frameworks/base,
and external/sepolicy are affected.  The CLs need to be approved
together, lest the build break or the software fail to boot.

Round 2: use proper type (jsize) for iteration variable;  whitespace.

Round 3: indentation.

Bug: 12114500
Change-Id: I31d25831b477b076d980c531d57773f9c8c83261