[Screen share] Split system cast and share-to-app perm dialog classes.

For b/352327853, the UX for the system cast permission dialog and the
share-to-app permission dialog will diverge even further from where they
are now. We're currently using a single DialogDelegate class to show
both types of dialogs. This CL splits that class into two classes to
make the rest of b/352327853's implementation easier.

Bug: 352327853
Flag: NONE refactor
Test: Share screen to app -> verify dialog has correct strings for
single app and entire screen, start share from dialog
Test: Cast screen to other device -> verify dialog has correct strings
for single app and entire screen, start share from dialog
Test: atest SystemCastPermissionDialogDelegateTest
ShareToAppPermissionDialogDelegateTest
Change-Id: I49217daca7bc779d724492f3457c2479c8631cc1

Change-Id: Ie16de17ff4bdaba9ea21a4261fd3e49cac303cdd