Demote Lint FlaggedApi violations to warnings

Lint FlaggedApi violations were recently promoted to errors. In most cases,
existing violations have been baselined. However, some projects still have
violations that have not been addressed, and baselining these has proven
difficult.

As a step towards a fully baselined (or better, fixed) codebase, demote Lint
FlaggedApi violations to warnings for the remaining projects.

Bug: 303434307
Test: presubmit
Flag: EXEMPT not possible to flag Android.bp changes
Change-Id: I84a309f52c828e72e8bcdc43c7492a451d25b87f