OPP: Ignore back press in Incoming alert screen

Use case:
1. Receive incoming file from remote
2. Tap on incoming notification to view incoming alert screen.
3. Press back button
4. perform above use case multiple times
5. Drag status bar and click on accept notification

Error:
Incoming notification not getting clear after click
on Accept.

Fix:
Avoid updating incoming status as hidden, and ignore
back press event in Incoming alert window.

Test: Able to receive file successfully after above usecase

Fixes: 77182502
Change-Id: I87e2077a5357744d8aa99b9d9c8103ba6acbedca