Mark @FlaggedApi flags as exported

An aconfig flag that is used together with @FlaggedApi must be marked as
`is_exported: true` to guarantee that the auto-generated lookup code
checks the actual flag value (instead of using a hard-coded value set at
compile time). This is important when the API is called across aconfig
container boundaries (e.g. a mainline module calling code on the system
partition).

Mark all non-exported flags used with @FlaggedApi as exported.

The "all @FlaggedApi flags are exported" invariant should be checked at
build time; this will be added in future CLs.

Bug: 378061535
Test: treehugger
Flag: EXEMPT can't flag changes to flag declarations
Merged-In: If45930e4afdcc87e374679ea73502651ce2de445
Change-Id: If45930e4afdcc87e374679ea73502651ce2de445