Android.bp: set sepolicy version for use by init

Init needs to be aware of the policy version defined in sepolicy
for on-device compilation.

Bug: 124499219
Test: build and boot a device. Try both precompiled and on-device
compiled policy.

Change-Id: Iba861aeb4566405aedcbe3c2bad48e1e50126370
diff --git a/Android.bp b/Android.bp
index eeb6f6c..256262b 100644
--- a/Android.bp
+++ b/Android.bp
@@ -12,6 +12,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+cc_defaults { name: "selinux_policy_version", cflags: ["-DSEPOLICY_VERSION=30"], }
+
 se_filegroup {
     name: "26.0.board.compat.map",
     srcs: [