fix(android): prevent screen from turning off during active video calls in example application

[mechtech-mind]

This PR fixes an issue in the Android example application where the device screen turns off automatically during an active video call due to screen timeout.

Problem

During an ongoing LiveKit call on Android, the screen is not kept awake, which leads to:

• Poor user experience during video calls
• Interrupted visibility when no user interaction occurs

Solution

This PR adds native Android window flag handling in MainActivity to ensure the screen remains active while a call is in progress:

• Uses native flags to prevent screen timeout
• Uses native flags as well to handle lock screen visibility

The flags are:

• Enabled when the room is active
• Cleared when the room is disposed

[mechtech-mind]

@hiroshihorie is it ok?

[mechtech-mind]

yeah looks ok