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
Change-Id: Id69176ae7dd29c0dcf9cbeaf9979028020c3a9f3

Author: amhk

services/surfaceflinger/surfaceflinger_flags_new.aconfig

@@ -31,6 +31,7 @@ flag {
   description: "New SDK Surface#setFrameRate API and Surface.FrameRateParams for Android 16"
   bug: "356987016"
   is_fixed_read_only: true
+  is_exported: true
 } # arr_setframerate_api
 
 flag {
@@ -39,6 +40,7 @@ flag {
   description: "New SDK SurfaceControl.Transaction#setFrameRate API for Android 16"
   bug: "356987016"
   is_fixed_read_only: true
+  is_exported: true
 } # arr_surfacecontrol_setframerate_api
 
 flag {