Use MappedFile for mmap-related operations in CursorWindow

MappedFile from libbase can encapsulate all mmap and munmap operations.
The advantage to MappedFile is that it is supported on all host
platforms, including Windows. This change is a no-op in terms of
functionality.

Test: Roboelctric SQLiteDatabaseTest
Bug: 317884162
Change-Id: I77b6b5481b64a930ecd2bd5ddf3692e54be60751