Ask proguard to keep SystemInstaller observers

Proguard removed the method org.fdroid.fdroid.installer.SystemInstaller$PackageInstallObserver$packageInstalled,
resulting in an AbstractMethodError crash.
Fixes issue [79](https://gitlab.com/fdroid/fdroidclient/issues/79).

proguard-project.txt

@@ -11,3 +11,9 @@
 # by certain certificates.
 #   Reference: https://gitlab.com/fdroid/fdroidclient/issues/88
 -keep class org.spongycastle.**
+
+# This keeps class members used for SystemInstaller IPC.
+#   Reference: https://gitlab.com/fdroid/fdroidclient/issues/79
+-keepclassmembers class * implements android.os.IInterface {
+    public *;
+}