data transfer: auto-accept and show images

* save files to private app storage
* auto-accept files < 20MB with image extension (jpg, jpeg, png)
* add ability to take picture directly from the conversation activity
 This required a FileProvider to be added.
* automatically show images directly in the conversation
* add simple MediaViewer activity to show full-screen media,
 making use of the zoomage library.

Change-Id: I7949094491ca928930cc3ad1e4519513e1883eca
diff --git a/ring-android/app/build.gradle b/ring-android/app/build.gradle
index 8f9a75b..39f300d 100644
--- a/ring-android/app/build.gradle
+++ b/ring-android/app/build.gradle
@@ -116,6 +116,8 @@
     // RxAndroid
     implementation 'io.reactivex.rxjava2:rxandroid:2.0.2'
 
+    implementation 'com.jsibbold:zoomage:1.1.0'
+
     withFirebaseImplementation 'com.google.firebase:firebase-messaging:11.8.0'
 }