Accept starting_at_board_api macro in service contexts

The macro requires the property target_board_api_level to be set.
Similary to policyConf, add a property to selinuxContext.

Bug: 367249722
Test: build with a call to starting_at_board_api in service_contexts.
Change-Id: I9b4b975a926a41f49138ca78c36b91310f57b586
diff --git a/build/soong/selinux_contexts.go b/build/soong/selinux_contexts.go
index d4c81e8..fd1cd34 100644
--- a/build/soong/selinux_contexts.go
+++ b/build/soong/selinux_contexts.go
@@ -46,6 +46,10 @@
 
 	// Make this module available when building for recovery
 	Recovery_available *bool
+
+	// Board api level of policy files. Set "current" for RELEASE_BOARD_API_LEVEL, or a direct
+	// version string (e.g. "202404"). Defaults to "current"
+	Board_api_level *string
 }
 
 type seappProperties struct {
@@ -288,6 +292,7 @@
 		Tool(ctx.Config().PrebuiltBuildTool(ctx, "m4")).
 		Text("--fatal-warnings -s").
 		FlagForEachArg("-D", ctx.DeviceConfig().SepolicyM4Defs()).
+		Flag(boardApiLevelToM4Macro(ctx, m.properties.Board_api_level)).
 		Flags(flagsToM4Macros(flags)).
 		Inputs(inputsWithNewline).
 		FlagWithOutput("> ", builtContext)