Package.Spec.SyncPeriod should be respected when PackageInstall.Spec.SyncPeriod is nil

**What steps did you take:**
When applying a PackageInstall CR with Spec.SyncPeriod == nil.

**What happened:**
The generated App.Spec.SyncPeriod is 10min.

**What did you expect:**
Package.Spec.Template.Spec.SyncPeriod (if not nil) should be respected when PackageInstall.Spec.SyncPeriod is nil.
Because each Package knows its proper SyncPeriod and deploy.kapp.RawOptions.'--wait-timeout=30s', unless PackageInstall CR explicitly sets the SyncPeriod.

**Anything else you would like to add:**
Here is the code causing the issue: https://github.com/vmware-tanzu/carvel-kapp-controller/blob/v0.36.1/pkg/packageinstall/app.go#L46-L52

**Environment:**

- kapp Controller version: v0.36.1 and develop branch

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