Replace old GrantPermissionsViewModel with new

This also moves the actual prompt -> button conversion logic to
GrantPermissionsActivity, where it belongs. The ViewModel itself will be
renamed in a followup CL. It is not renamed here for ease of viewing the
diff

Bug: 284183336
Test: atest CtsPermissionUiTestCases
Change-Id: I43b1e3eadbc9388e4c99c956dce534c50074f8e1