REF: avoid access to _mgr.blocks -> _mgr._is_single_block
@jbrockmendel a small follow-up on https://github.com/pandas-dev/pandas/pull/36873. It keeps the same spirit of that PR (I think), but just replacing the access of `len(self._mgr.blocks)` with an existing attribute on the manager (could also remove the leading underscore on the attribute). 
(this makes it easier to override this attribute in the ArrayManager, and limits the access to the blocks outside of the internals)