Move ViewGroup#mChildren[Count] to dark-grey list

The CL moves mChildren and mChildrenCount in ViewGroup to the dark-grey
list of APIs, disabling access to them for apps targeting Q. Developers
should probably use ViewGroup#getChildCount() and ViewGroup#getChildAt()
which exist since public API 1.

Bug: 117521014
Bug: 117521406
Test: atest core/tests/coretests/src/android/view/
Change-Id: I14d3ebd1b16edc92cd7b370404b1f05cd304ab7d