exports: Add assertions to distinguish between mode representations

When we're talking about a "mode", sometimes we mean a
FlatpakFilesystemMode, sometimes we mean a FlatpakFilesystemMode that
must be strictly greater than NONE, and sometimes we're willing to
accept the FAKE_MODE constants too.

Signed-off-by: Simon McVittie <smcv@collabora.com>
(cherry picked from commit 115d82e6ff)
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