Fix aspect ratio for letterboxed activity in `LetterboxAppHelper`

Set the non-resizeable portrait activity used by the
`LetterboxAppHelper` to have a min aspect ratio to ensure the activity
is always letterboxed in all orientations.

Flag: NONE(bug fix)
Fixes: 371124043
Test: atest WMShellFlickerTestsAppCompat:QuickSwitchLauncherToLetterboxAppTest
Change-Id: Id84a4b94fc8c66ab48d0e0d274b559298f7fe838