No sound and vibration in push messages

[lenzch]

When using Android devices with version >= 8 neither sound nor vibration effects are applied, when a push notification is received on the default channel.

The root cause seems to be, that the default channel created by the library has IMPORTANCE_LOW assigned during creation.
This prevents any 'intrusive' effects.

Please change it to at least IMPORTANCE_DEFAULT

Code for referene:

bms-clientsdk-android-push/lib/src/main/java/com/ibm/mobilefirstplatform/clientsdk/android/push/api/MFPPushIntentService.java

Line 294 in 0beaa8c

int importance = NotificationManager.IMPORTANCE_LOW;

[lenzch]

Please see #138 for change