Fix debugger crashes in presence of proxy objects.

Fix debugger crashes in presence of proxy objects.

Fix ClassHelper::NumDirectInterfaces to use IfTable::Count instead of the
iftable array's length (which is twice the interface count). This happens when
processing ReferenceType::Interfaces command and was causing a crash.

Return ABSENT_INFORMATION error code when we're asked for the source file of a
proxy class. Otherwise we call ClassHelper::GetSourceFile which expect the
class has a corresponding ClassDef item in the DEX file which is not true for
proxy classes.

Add new proxy_test to check ClassHelper works correctly with proxy classes.

Bug: 13426918
Change-Id: I5c1212b1a697dd7dc1ab18e99552ee113c533a5a