dexlayout: ProcessOffset in WriteHiddenapiClassData

WriteHiddenapiClassData would not call ProcessOffset before starting to
write data. This is only a problem if compute_offsets_ is false as it
would continue writing where previous section left off and that place
not being the end of the file.

This fixes dexlayout_test gtest.

Bug: 123150330
Test: m test-art-host-gtest-dexlayout_test
Change-Id: I552122d0968584db4a53d7219986f9693210b1e1