Cleanup license metadata in external/rust/crates/dashmap.

Add package and license module in Android.bp. File LICENSE is from upstream project, so use it in Android.bp.

Bug: 346390141
Test: CIs
Change-Id: Ide0f136da8419c7755b5ad5fddf80ccfcee51cf9

Android.bp

diff --git a/Android.bp b/Android.bp
index 78a13a6..c0a73bb 100644
--- a/Android.bp
+++ b/Android.bp
@@ -3,7 +3,21 @@
 // because the changes will be overridden on upgrade.
 // Content before the first "rust_*" or "genrule" module is preserved.
 
-// TODO: Add license.
+package {
+    default_applicable_licenses: ["external_rust_crates_dashmap_license"],
+}
+
+license {
+    name: "external_rust_crates_dashmap_license",
+    visibility: [":__subpackages__"],
+    license_kinds: [
+        "SPDX-license-identifier-MIT",
+    ],
+    license_text: [
+        "LICENSE",
+    ],
+}
+
 rust_test {
     name: "dashmap_test_src_lib",
     host_supported: true,