Fix board_api_level comments and implementation

We accidentally ignored board_api_level property on aosp/3252595. Fix
it.

Also this cleans up the unnecessary default module
se_policy_conf_public_flags_defaults.

Bug: 358516352
Test: TH
Change-Id: I786cae0b12d2629d26ce4335866df364c218b6d0
diff --git a/build/soong/policy.go b/build/soong/policy.go
index 29685ea..d490845 100644
--- a/build/soong/policy.go
+++ b/build/soong/policy.go
@@ -91,8 +91,8 @@
 	// Desired number of MLS categories. Defaults to 1024
 	Mls_cats *int64
 
-	// Board api level of policy files. Set "vendor" for RELEASE_BOARD_API_LEVEL, "system" for
-	// turning off the guard, or a direct version string (e.g. "202404"). Defaults to "system"
+	// 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
 }
 
@@ -225,7 +225,12 @@
 }
 
 func (c *policyConf) boardApiLevel(ctx android.ModuleContext) string {
-	return ctx.Config().VendorApiLevel()
+	level := proptools.StringDefault(c.properties.Board_api_level, "current")
+	if level == "current" {
+		return ctx.Config().VendorApiLevel()
+	} else {
+		return level
+	}
 }
 
 func findPolicyConfOrder(name string) int {