Fix AHI HRIT file patterns so area's ID is correct

[djhoese]

This is a fix after #524 where JMA-based file patterns need to have an area pattern to have the produced AreaDefinition with the proper name. In the ahi_hrit reader this was not done and the area id was UNKNOWN instead of the desired FLDK.

• Tests added
• Tests passed
• Passes git diff origin/master -- "*py" | flake8 --diff

[coveralls]

Coverage remained the same at 76.694% when pulling 454cb19 on djhoese:bugfix-ahi-area into 49027e8 on pytroll:master.

[codecov]

Codecov Report

Merging #550 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #550   +/-   ##
=======================================
  Coverage   76.69%   76.69%           
=======================================
  Files         136      136           
  Lines       18961    18961           
=======================================
  Hits        14542    14542           
  Misses       4419     4419

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