graphics: make allocator passthrough library header-only

android.hardware.graphics.allocator@2.0-passthrough should be a
header-only library to be fully reusable by vendor HALs.

This also allows us to switch from virtual inheritance to templates,
which is more straightforward.  This changes nothing to the users
and we still have these relations

 - AllocatorHal is an abstract class to be implemented by vendors or
   the default implementations
 - Gralloc[01]Hal are our default implementations
 - Allocator implements HIDL IAllocator interface on top of
   AllocatorHal

What we do not like about virtual inheritance is that, given

  // abstract class B and D
  class B {
    virtual void foo() = 0;
    virtual void bar() = 0;
  };
  class D : public virtual B {
    // foo is superceded by fooEnhanced in D
    void foo() { fooEnhanced(); }
    virtual void fooEnhanced() = 0;
  };

  // an implementation of B
  class BImpl : public virtual B {
    void foo() {}
    void bar() {}
  };

  // an implementation of D on top of BImpl
  class DImpl : public virtual D, public virtual BImpl {
    void fooEnhanced() {}
  };

we get "no unique final overrider" becase both D and BImpl implement
foo.  With non-virtual inheritance, on the other hand, we get "DImpl
is abstract" because foo is still pure virtual implemented in DImpl.
Templates solve the issue by allowing

  namespace detail{
  template<typename T>
  class BImpl : public T { ... };

  template<typename T>
  class DImpl : public BImpl<T> { ... };
  } // namespace detail

  using BImpl = detail::BImpl<B>;
  using DImpl = detail::DImpl<D>;

Test: boots
Change-Id: Iccb513e4fc751e9a687a1ed2d9fb2192c8324a50