[base] Make MappedFile work with OS file handles

adb uses its own implementation of Windows HANDLE->int mapping,
and it doesn't play well with _get_osfhandle() function used in
MappedFile::FromFd().

This CL adds another function that accepts raw handle, so adb
can pass it directly

+ make constant functions 'const'
+ make the MappedFile movable, as nothing prevents it from being
  one

Test: libbase_test

Change-Id: Ifde4a4094b910e9c7b431126ecf3fef5aa3bb4a6