Move non-updatable-api/ to core/api/

This achieves two things:
- it's been noted that non-updatable-api is a confusing term as it has
  to be updated every time the API changes
- the api-review logic trigers on txt files in subdirs named 'api',
  which non-updatable-api does not trigger on

I will (less urgently) try to get rid of the term "non-updatable" from
other places too, perhaps going for "non-module" or "core" as a
replacement.

Bug: 171029603
Test: m checkapi
Change-Id: I843efb4880b6caaf3872e9b0918fd710fb93d979

StubLibraries.bp

@@ -152,8 +152,8 @@ droidstubs {
     args: metalava_framework_docs_args,
     check_api: {
         current: {
-            api_file: "non-updatable-api/current.txt",
+            api_file: "core/api/current.txt",
-            removed_api_file: "non-updatable-api/removed.txt",
+            removed_api_file: "core/api/removed.txt",
         },
         last_released: {
             api_file: ":android-non-updatable.api.public.latest",
@@ -212,8 +212,8 @@ droidstubs {
     args: metalava_framework_docs_args + priv_apps,
     check_api: {
         current: {
-            api_file: "non-updatable-api/system-current.txt",
+            api_file: "core/api/system-current.txt",
-            removed_api_file: "non-updatable-api/system-removed.txt",
+            removed_api_file: "core/api/system-removed.txt",
         },
         last_released: {
             api_file: ":android-non-updatable.api.system.latest",
@@ -223,7 +223,7 @@ droidstubs {
         api_lint: {
             enabled: true,
             new_since: ":android-non-updatable.api.system.latest",
-            baseline_file: "non-updatable-api/system-lint-baseline.txt",
+            baseline_file: "core/api/system-lint-baseline.txt",
         },
     },
 }
@@ -295,8 +295,8 @@ droidstubs {
     args: metalava_framework_docs_args + priv_apps + module_libs,
     check_api: {
         current: {
-            api_file: "non-updatable-api/module-lib-current.txt",
+            api_file: "core/api/module-lib-current.txt",
-            removed_api_file: "non-updatable-api/module-lib-removed.txt",
+            removed_api_file: "core/api/module-lib-removed.txt",
         },
         last_released: {
             api_file: ":android-non-updatable.api.module-lib.latest",