Align entries in APEX at 4K boundary

Use -a option of signapk to specify the alignment to 4K. The default is
4.

Test: m apex.test; zipalign -c -v 4096 $(OUT)/system/apex/apex.test.apex
shows ok

Change-Id: I7b1a3d442b69b89838cf09451939cd23d5646bde

java/app_builder.go

@@ -32,10 +32,10 @@ var (
 	Signapk = pctx.AndroidStaticRule("signapk",
 		blueprint.RuleParams{
 			Command: `${config.JavaCmd} -Djava.library.path=$$(dirname $signapkJniLibrary) ` +
-				`-jar $signapkCmd $certificates $in $out`,
+				`-jar $signapkCmd $flags $certificates $in $out`,
 			CommandDeps: []string{"$signapkCmd", "$signapkJniLibrary"},
 		},
-		"certificates")
+		"flags", "certificates")
 
 	androidManifestMerger = pctx.AndroidStaticRule("androidManifestMerger",
 		blueprint.RuleParams{