Remove unnecessary explicit GC in ZygoteInit.

Removes > 1s of GC time and hundreds of GC during device boot.

Zygote size before: 8388608
Zygote size after: 8388608

No increase in zygote space size due to zygote compaction.

Bug: 16398684

Change-Id: I3c6f5a0b26a207ab39bb0a952400d7488e42f00c
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: indent change

Bug: 12114500
Change-Id: I090402136a8a8b7d6aad6eb153026e85d7cf6ad3