Avoid SetErrorHandler mutating FixtureFactory

A FixtureFactory is supposed to be immutable to allow them to be safely
shared but unfortunately the implementation of SetErrorHandler broke
that constraint. That made it very easy to mistakenly add an error
handler specific to a test to a shared factory breaking other tests
that use that factory.

This change causes SetErrorHandler to create a new instance of the
factory to avoid that.

Bug: 181070625
Test: m nothing
Change-Id: Ia5356a04189099c88880a2a521af29ab72560f30