default the prev session to be invalid in a new app install

Background:
Each session has an `isValid` property that is read at the start of a potential new session. If this value read is `false`, then a new session will start and its property is set to `true`. If this value read is `true`, that means we continue on this same previous session.
At the end of the session, this property will be set to `false`.

Problem:
On a brand new app install, we were defaulting this value to `true`, which means the logic to trigger a new session will not happen, since the SDK will believe it should continue from a previous session. However, this is the first time the app is opened after a new install.

Solution:
By defaulting instead to `false`, we ensure that this first app open will start a new session. In addition, this will trigger a `TRACK_SESSION_START` operation and ask the server to `refresh_device_metadata`, updating country and IP for this user.

OneSignalSDK/onesignal/core/src/main/java/com/onesignal/session/internal/session/SessionModel.kt

@@ -19,7 +19,7 @@ class SessionModel : Model() {
      * Whether the session is valid.
      */
     var isValid: Boolean
-        get() = getBooleanProperty(::isValid.name) { true }
+        get() = getBooleanProperty(::isValid.name) { false }
         set(value) {
             setBooleanProperty(::isValid.name, value)
         }