Add is_exported field to aconfig.proto

When a flag is exported, it can be read by code built in other containers. By
default flags are not exported. Set this to true if your flag is gating
a public API which may be called from other containers (typically, a
flag used in @FlaggedAPI annotations).

Test: atest aconfig.test
Bug: 311152500
Change-Id: I53e3ed18b96c1518f04172d5933ef96b41ccda7e