Fix preserveLegacyExternalStorage logic

preserveLegacyExternalStorage manifest flag was introduced in
Ifd3410ed1a60f4c0e8414fce904139b539e13ad8 and allowed any app
regardless of targetSdk preserve legacy storage.

I31d4b989aea3b73efb3a3634073ee06b88e3ee97 changed the condition to
only allow targetSdk <= Q preserve legacy. However, it interestingly
didn't break behavior (cos we have test to catch this
RestrictedStoragePermissionTest#testStorageTargetingSdk30CanPreserveLegacyOnUpdateFromLegacy)

Here's why: In SoftRestrictedPermissionPolicy, the preserve flag
effectively only changes the behavior of the
mayDenyExtraAppOpIfGranted method. So even though the
mayAllowExtraAppOp method behavior was changed, this wasn't needed to
be true for the preserve flag to be effective, since it was only
'keeping' any existing legacy appop based on the result of
mayDenyExtraAppOpIfGranted.

There's no good regression test to add for this, since in theory
behavior actually didn't regress. In any case, we now fix the
condition.

Test: atest RestrictedStoragePermissionTest
Bug: 151393596
Change-Id: I1f4fd8c7b9fab72c2649f6c8e8fd91edc12d5cb4