Add uniffi_core crate

There is no `Android.bp` file in the repository, so this is safe. The
`Android.bp` file will be added later in a regular CL.

Bug: 330712502
Test: Presubmit
Change-Id: I606e4b540535cf84ef8293caafef092dbe8fa230
diff --git a/default.xml b/default.xml
index 81d8e75..c30b907 100644
--- a/default.xml
+++ b/default.xml
@@ -818,6 +818,7 @@
   <project path="external/rust/crates/unicode-segmentation" name="platform/external/rust/crates/unicode-segmentation" groups="pdk" />
   <project path="external/rust/crates/unicode-width" name="platform/external/rust/crates/unicode-width" groups="pdk" />
   <project path="external/rust/crates/unicode-xid" name="platform/external/rust/crates/unicode-xid" groups="pdk" />
+  <project path="external/rust/crates/uniffi_core" name="platform/external/rust/crates/uniffi_core" groups="pdk" />
   <project path="external/rust/crates/uniffi_meta" name="platform/external/rust/crates/uniffi_meta" groups="pdk" />
   <project path="external/rust/crates/unsafe-libyaml" name="platform/external/rust/crates/unsafe-libyaml" groups="pdk" />
   <project path="external/rust/crates/untrusted" name="platform/external/rust/crates/untrusted" groups="pdk" />