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

apex/apex.go

@@ -662,6 +662,7 @@ func (a *apexBundle) buildUnflattenedApex(ctx android.ModuleContext, keyFile and
 		Input:       unsignedOutputFile,
 		Args: map[string]string{
 			"certificates": strings.Join([]string{certificate.Pem.String(), certificate.Key.String()}, " "),
+			"flags":        "-a 4096", //alignment
 		},
 	})
 }