Move StaggeredGridView into ex from v4 support library

Move this class because it's still experimental and
undergoing development. Below is the git history
log of the original file.

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commit b469af6dc2f8cda4020a78fb4582c1483089fd6e
Author: Adam Powell <adamp@google.com>
Date:   Thu Apr 12 10:58:19 2012 -0700

    Post invalidations on the animation tick if available for
    StaggeredGridView/ViewPager.

    Fix interaction with nested scrolling in ViewPager.

    Finish settling a ViewPager that is close enough to the end of a fling
    when touched.

    Change-Id: Ia68be871c92cdb448d4030fdc4b806c78ea69c1a

commit 77e2f963d4bcac5e0a4704cd4ea684b04735053e
Author: Katherine Kuan <katherinekuan@google.com>
Date:   Mon Apr 2 14:46:54 2012 -0700

    Reset item tops and bottoms for StaggeredGridView

    When the StaggeredGridView is going to display
    a completely new set of data, offer a reset method
    so item top and bottom information is cleared.

    Fix NPE when adapter notifyDataSetChanged() is
    called.

    Bug: 6282841
    Change-Id: If1aee80358e32420d2b3a0d4e48ede39002d650a

commit 25df93b813befb678835eb040c8158bb5aa8fe8d
Author: Adam Powell <adamp@google.com>
Date:   Mon Mar 19 20:46:57 2012 -0700

    Fix a bug with changing the column count in StaggeredGridView.

    Change-Id: I7aa6544e03e4c7426c96a906f7c78b91d2e8ebb6

commit 121dba12686f0ff01cfe348c43a4f6f0edc18477
Author: Adam Powell <adamp@google.com>
Date:   Mon Mar 19 20:22:22 2012 -0700

    Properly handle top padding in StaggeredGridView while freshly populating items.

    Change-Id: I6aa920320217017bc8720a4170739aee06b17537

commit 346e2f2390f0d743fd10e7d01a015df6b32292cd
Author: Adam Powell <adamp@google.com>
Date:   Tue Feb 28 10:13:02 2012 -0800

    StaggeredGridView and supporting functionality

    Stable IDs are not yet supported.

    Move/rename HCSparseArray => SparseArrayCompat; make it public.

    Add some new features to ViewCompat.

    Add ScrollerCompat; leave it package-private for now as it needs
    a reasonable fallback implementation for new methods.

    Change-Id: I87d6952ef2c7748a40558759372a2525d6a52cf0

Change-Id: I794a7b9666b0414d75a99ff2a5f939193b78026a