Allow libcore to have NewApi warnings

The majority of libcore NewApi warnings come from references to new
methods, constants, fields added in a file and then referenced in the
same source file. Maintaining suppressions is not scalable: they need
to work across multiple API versions in different branches.

Bug: 208656169
Test: TH
Change-Id: If5e746f25f1bd34931ee8c89b363aa92c81e23b3