private void addAlwaysAllowedCallerPackage(final String pkg) {
		final SharedPreferences preferences = PreferenceManager.getDefaultSharedPreferences(this);
		final Set<String> pkgs = new HashSet<>(requireNonNull(preferences.getStringSet(PREF_KEY_DIRECT_INSTALL_ALLOWED_CALLERS, Collections.emptySet())));
		pkgs.add(pkg);
		preferences.edit().putStringSet(PREF_KEY_DIRECT_INSTALL_ALLOWED_CALLERS, pkgs).apply();
	}
App installer with following capabilities:
<ul>
<li> Install APK from Mainland into Island, and vice versa.
<li> Install app without user consent
</ul>
Created by Oasis on 2018-11-12.
installer/src/main/java/com/oasisfeng/island/installer/AppInstallerActivity.java