Avoid allocating OatFile::OatClass on the heap.

Avoid allocating a BitVector for OatFile::OatClass::bitmap_
with kOatClassSomeCompiled methods. That makes the OatClass
copy-constructible as it doesn't own any memory. We use that
in OatFile::OatDexFile::GetOatClass() to return the result
by value thus avoiding one or two heap allocations per call.

Change-Id: Ic7098109028a5b49e39ef626f877de86e732ed18