Replacing @Nullable with @RecentlyNullable for apis annotated since API 28

(temporarily). This causes Metalava to downgrade @Nullable/@NonNull to
@RecentlyNullable/@RecentlyNonNull for now.

This should provide more time to migrate calling code.

Note that we're still checking compatibility against API 29.

Test: m api-stubs-docs # and unzip out/target/common/docs/api-stubs-docs-stubs.srcjar \
      and notice that @Nullable/@NonNull are replaced by @RecentlyNullable/@RecentlyNonNull

Test: m checkapi

Test: echo "package samplePackage { public class sampleClass { } }" >> prebuilts/sdk/29/public/api/android.txt \
      && m checkapi # and notice that it correctly reports an error

Bug: 140127380
Change-Id: I616438183ef80d1da7cbbe1c0eb01d1f71eaad8c