Account for multiple BOARD_PLAT_PUBLIC[PRIVATE]_SEPOLICY_DIR dirs

After https://android-review.googlesource.com/688488
BOARD_PLAT_PUBLIC[PRIVATE]_SEPOLICY_DIR can now specify multiple
directories.

Bug: n/a
Test: build sepolicy
Change-Id: I838c76736a3f1bd8759e24739cf82fea33a531b4
diff --git a/android/variable.go b/android/variable.go
index 6599ca3..af414cb 100644
--- a/android/variable.go
+++ b/android/variable.go
@@ -210,10 +210,10 @@
 
 	PgoAdditionalProfileDirs []string `json:",omitempty"`
 
-	BoardVendorSepolicyDirs     []string `json:",omitempty"`
-	BoardOdmSepolicyDirs        []string `json:",omitempty"`
-	BoardPlatPublicSepolicyDir  string   `json:",omitempty"`
-	BoardPlatPrivateSepolicyDir string   `json:",omitempty"`
+	BoardVendorSepolicyDirs      []string `json:",omitempty"`
+	BoardOdmSepolicyDirs         []string `json:",omitempty"`
+	BoardPlatPublicSepolicyDirs  []string `json:",omitempty"`
+	BoardPlatPrivateSepolicyDirs []string `json:",omitempty"`
 
 	VendorVars map[string]map[string]string `json:",omitempty"`
 }