OPP: Fix Window leak in Alert Activity.

Use case:
1) Send or receive file
2) During transfer open on transfer progress
3) do above steps multiple times

Observed Results:
Observed Window Leaked warnings in
Bluetooth Context.

Fix:
Override onDestroye method in custom alert activity
and dismiss Alert Dialog instance to prevent window leaked
issue in Bluetooth.

Bug: 152473345
Test: Performed above use case, issue not seen
Change-Id: Ieb00150fa7ace01ae86238d0abe7412d1e5e524b