Make ext build against core.platform.api.stubs

The ext target needs to be switched from core_current to depending on
core.platform.api.stubs (the default when no sdk_version is specified
and no_frameworks_libs = true) as it statically includes
libphonenumber-platform which itself needs to depend on
core.platform.api.stubs as it needs access to the
dalvik.annotation.compat.UnsupportedAppUsage annotation.

Without this change modifying the libphonenumber-platform target to
depend on core.platform.api.stubs causes a build failure.

Tested by changing libphonenumber-platform target to depend on
core.platform.api.stubs and running make checkbuild.

Bug: 117818301
Test: see above
Change-Id: I2b9154d22b67aafb57493b41b527818c37212c34
diff --git a/Android.bp b/Android.bp
index 8b60745..d8bbf4c 100644
--- a/Android.bp
+++ b/Android.bp
@@ -816,7 +816,7 @@
 java_library {
     name: "ext",
     installable: true,
-    sdk_version: "core_current",
+    no_framework_libs: true,
     static_libs: [
         "libphonenumber-platform",
         "nist-sip",