Remove @FlaggedApi from supplementalDescription

and fix @FlaggedApi in public-final.xml (it was missing qoutes for the
supplemental_description flag)

This was confusing aapt2 when creating android.R.styleable, it was
rejecting this resoucre if the flag was READ_WRITE, which it always is
for trunk_staging

RESOURCES_FINALIZATION

Flag: EXEMPT Baklava Finalization
Bug: 393356045
Bug: 396046850
Bug: 396104948
Test: m out/soong/.intermediates/frameworks/base/api/ds-static-docs/android_common/ds-static-docs-docs.zip
Change-Id: I4bf244f5a2eb17a6cad06386e845efdd19028d49