Support building mixed versions of sepolicy

Now newer system policy and older vendor policy can be built together by
setting following variables:

- BOARD_SEPOLICY_VERS
- BOARD_REQD_MASK_POLICY (copy of older system/sepolicy/reqd_mask)
- BOARD_PLAT_VENDOR_POLICY (copy of older system/sepolicy/vendor)
- BOARD_(SYSTEM_EXT|PRODUCT)_(PUBLIC|PRIVATE)_PREBUILT_DIRS (copy of
  older system_ext and product policies)

Bug: 168159977
Test: try normal build and mixed build
Test: boot and check selinux denials
Change-Id: I20e1986cc1c877f5e3a7965e03bd8ade84fd7230

android/variable.go

@@ -311,10 +311,14 @@ type productVariables struct {
 
 	BoardVendorSepolicyDirs      []string `json:",omitempty"`
 	BoardOdmSepolicyDirs         []string `json:",omitempty"`
+	BoardReqdMaskPolicy          []string `json:",omitempty"`
 	SystemExtPublicSepolicyDirs  []string `json:",omitempty"`
 	SystemExtPrivateSepolicyDirs []string `json:",omitempty"`
 	BoardSepolicyM4Defs          []string `json:",omitempty"`
 
+	BoardSepolicyVers       *string `json:",omitempty"`
+	PlatformSepolicyVersion *string `json:",omitempty"`
+
 	VendorVars map[string]map[string]string `json:",omitempty"`
 
 	Ndk_abis               *bool `json:",omitempty"`