Create feature factory interface & initial impl.

Creates the abstract class FeatureFactory and an AOSP implementation
called Factory.

Also creates a static library for generating logtags since we need to use the code generated
by the logtags in multiple packages now.

BUG: 27751878
Change-Id: I88d826333642d3efc252134c4facb7b1ca014f32
(cherry picked from commit 867bb9c07ab332fd9be67d1eeee3db796d9e0ffa)
diff --git a/proguard.flags b/proguard.flags
index 3c3300e..f3c218a 100644
--- a/proguard.flags
+++ b/proguard.flags
@@ -17,6 +17,7 @@
 -keep class com.android.settings.users.*
 -keep class com.android.settings.nfc.*
 -keep class com.android.settings.notification.*
+-keep class com.android.settings.overlay.FeatureFactoryImpl
 -keep class com.android.settings.accessibility.*FragmentForSetupWizard
 -keep class com.android.settings.display.*FragmentForSetupWizard