Add container field to flag_declarations

A container is software which is always built in its entirety using the
same build environment. In particular, all of its parts are built using
the same build-time default flag values. In addition, containers are
always installed as a single unit.

Bug: 312696545
Test: atest aconfig.test && m all_aconfig_declarations
Change-Id: I2ef3db836c4456f4f4fb5c066edf9094e38f89cc
diff --git a/tools/aconfig/protos/aconfig.proto b/tools/aconfig/protos/aconfig.proto
index 9e193ec..1a80b04 100644
--- a/tools/aconfig/protos/aconfig.proto
+++ b/tools/aconfig/protos/aconfig.proto
@@ -46,6 +46,7 @@
 message flag_declarations {
   optional string package = 1;
   repeated flag_declaration flag = 2;
+  optional string container = 3;
 };
 
 message flag_value {
@@ -79,7 +80,7 @@
   repeated tracepoint trace = 8;
   optional bool is_fixed_read_only = 9;
   optional bool is_exported = 10;
-
+  optional string container = 11;
 }
 
 message parsed_flags {