Refactoring before adding stricter validity checking

Refactoring before adding stricter validity checking.
The tz update behavior needs a way to validate tzdata
files and the current code structure doesn't make that
easy. This commit is mostly rearranging the existing
code to provide a mechanism for loading a single file.
The only functional change intended is to add a
close() call to close the MemoryMappedFile when a file
has been found to be invalid.

Bug: 31008728
Test: CtsLibcoreTestCases
Change-Id: I3c016f0a489319d93d5621ed3de2763f46ea534c