Not implement Iterator for OverlayManagerTransaction

OverlayManagerTransaction.Request is set by apps. The apps know the
content of requests. There is no need to public
OverlayManagerTransaction.Request. However, OverlayManagerTransaction
implements Iterator and it makes the public API more complicated.

This patch makes OverlayManagerTransaction not to implement
Iterator and make the class to be final.

Bug: 205919743
Test: atest \
        OverlayHostTests \
        OverlayDeviceTests \
        SelfTargetingOverlayDeviceTests \
        OverlayRemountedTest \
        FrameworksServicesTests:com.android.server.om \
        CtsContentTestCases:android.content.om.cts \
        idmap2_tests
Change-Id: I6e0f4a8f28d5ca1ead0de8a9037d0180ab7c5a2d