Show connected display in cast and screenshare dialogs

Similarly to the recording dialog, also add the list of connected
display for cast and screen share permission dialog.

The CL also move the getConnectedDisplay() method into the Util file for
DRY principles.

Flag: com.android.media.projection.flags.media_projection_connected_display_screen_sharing
Bug: 406217983
Test: atest SystemUITests:com.android.systemui.mediaprojection.permission.ShareToAppPermissionDialogDelegateTest
Change-Id: I52160852b5ad1d3958e6aec051f06f378cdf52cc