Fix copy / move behaviour of Maps object.

Currently, moving or copying a Maps object leads to double free of MapInfo.

Even moving a Maps object  did not prevent this, as after a move
the object only has to be in an "unspecified but valid state", which can
be the original state for a vector of raw pointers (but not for a vector
of unique_ptrs).

Changing to unique_ptrs is the most failsafe way to make sure we never
accidentally destruct MapInfo.

Test: atest libuwindstack_test
      Failed LocalUnwinderTest#unwind_after_dlopen which also fails at master.

Change-Id: Id1c9739b334da5c1ba532fd55366e115940a66d3