Make imgdiag more useful for dirty page in zygote

1) Fixes dirty labels to make them more intuitive.  The previous wording
of the headings for dirty object dumps was confusing.  Make it more obvious
which are the differences only in the zygote and which are from the image (application).

2) Make tool work with only --zygote-diff-pid specified.

Bug: 62554875
Test: adb shell imgdiag --image-diff-pid=<app pid> --boot-image=/system/framework/boot.art
Change-Id: Id0585ec0c9de89a86761a5f494c0dff655c4e64f