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
Merged-In: I87e2077a5357744d8aa99b9d9c8103ba6acbedca
(cherry picked from commit 38a111dc71e8f6d3db2520f971b733b41a2ef7bd)