Target API check for margin params fix

Prior to N, our widgets were not converting MarginLayoutParams
properly between ViewGroups. The fix intrudced some issues in
older apps as the broken conversion code would hide developer
errors. This CL guards the change with a target API check so
that we don't affect older apps.

Bug: 30378230
Change-Id: I215281d261b553c3b4cedcd29ea0a861df809471