Fix crash in JDWP ReferenceType.SourceFile command

Updates Dbg::GetSourceFile to return ABSENT_INFORMATION error code when
Class::GetSourceFile returns nullptr. This happens if the class has no source
file information.

Updates Class:GetSourceFile to return nullptr for classes which have no
ClassDef item like generated classes. This allows to remove the IsProxyClass
test from Dbg::GetSourceFile. Adds this test in proxy_test.

Bug: 15426710
Change-Id: I019da4ced83778d5264484c43b225f8b5c95632e
3 files changed