Fix long GATT read operations

[beaufortfrancois]

We should not return an invalid offset when performing read GATT operations if it needs multiple reading due to a small MTU size (Mac)

Background: https://bugs.chromium.org/p/chromium/issues/detail?id=711286

R: @g-ortuno

[g-ortuno]

A couple of questions:

1. Since we are using value.length chances are we are sending more bytes than necessary, could we fix that? If not then please comment why we use the length.
2. I'm somewhat surprised of how we are using the API. I would have guessed that offset would indicate the Android API to only send the data starting from offset. The fact that we 1. send offset back and 2. only send part of the value is making me really suspicious. Could you confirm that simply removing this if statement isn't enough to fix the issue? If it's not enough then I think we can just make that the default behavior:

Log.d(TAG, "Value: " + Arrays.toString(value));
mGattServer.sendResponse(device, requestId, BluetoothGatt.GATT_SUCCESS, offset,
      // TODO: Comment why we use value.length
      Arrays.copyOfRange(value, offset, value.length));