use appropriate fallback chain for style

TextLayoutShaper was defaulting to the Regular-style fallback chain
for all unknown scripts. This became problematic in that the
codepoint->glyphID enumeration stage of shaping was always using the
Regular-style fallback chain for unknown scripts, and there's no
guarantee that glyph indices are compatible between fallback chains.

Defaulting to a style-appropriate fallback chain addresses this
issue, and probably unreported but related issues, as well.

Bug: 8189208
Change-Id: I6ecf531c74d71a8e4a5359d23439ccc950b0cf80