Calculate whether an app is an APK or not when updating repos.

This improves performance when we need to decide whether or not apps are
installed or not while scrolling through large lists.

Fixes #1143.

Also change Jackson tests to properly ignore App#isApk.

app/src/test/java/org/fdroid/fdroid/updater/IndexV1UpdaterTest.java

@@ -304,6 +304,7 @@ public class IndexV1UpdaterTest extends FDroidProviderTest {
                 "installedSig",
                 "installedVersionCode",
                 "installedVersionName",
+                "isApk",
                 "preferredSigner",
                 "prefs",
                 "TAG",
@@ -335,6 +336,7 @@ public class IndexV1UpdaterTest extends FDroidProviderTest {
                 "hash",
                 "hashType",
                 "incompatibleReasons",
+                "isApk",
                 "maxSdkVersion",
                 "minSdkVersion",
                 "nativecode",