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

Android.bp

@@ -931,7 +931,7 @@ stubs_defaults {
         "sdk-dir",
         "api-versions-jars-dir",
     ],
-    previous_api: ":last-released-public-api",
+    previous_api: ":last-released-public-api-for-metalava-annotations",
     merge_annotations_dirs: [
         "metalava-manual",
         "ojluni-annotated-sdk-stubs",
@@ -988,7 +988,7 @@ stubs_defaults {
     local_sourcepaths: frameworks_base_subdirs,
     installable: false,
     annotations_enabled: true,
-    previous_api: ":last-released-public-api",
+    previous_api: ":last-released-public-api-for-metalava-annotations",
     merge_annotations_dirs: [
         "metalava-manual",
         "ojluni-annotated-sdk-stubs",
@@ -1324,7 +1324,7 @@ droidstubs {
     installable: false,
     sdk_version: "core_platform",
     annotations_enabled: true,
-    previous_api: ":last-released-public-api",
+    previous_api: ":last-released-public-api-for-metalava-annotations",
     merge_annotations_dirs: [
         "metalava-manual",
         "ojluni-annotated-sdk-stubs",