Move String::SizeOf<> to string.h .

string.h was already indirectly (through class.h) including
everything needed for String::SizeOf<>, so move it to the
string.h and make those includes direct. Then we can change
the object-inl.h to include only string.h, reducing the
number of indirectly included files.

Test: m test-art-host-gtest
Test: m testrunner.py --host --optimizing
Change-Id: I51c462c034f205498c539abe6d888be9738d4a6e