Prefer `.first()` to `.get(0)`

```
error: accessing first element with `key_descriptors.get(0)`
   -->
system/security/keystore2/tests/keystore2_client_list_entries_tests.rs:143:27
    |
143 |                 let key = key_descriptors.get(0).unwrap();
    |                           ^^^^^^^^^^^^^^^^^^^^^^ help: try:
`key_descriptors.first()`
    |
    = help: for further information visit
https://rust-lang.github.io/rust-clippy/master/index.html#get_first
    = note: `-D clippy::get-first` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::get_first)]`
```

Bug: http://b/321303117
Test: toolchain/android_rust/test_compiler.py --prebuilt-path dist/rust-dev.tar.xz --target aosp_cf_x86_64_phone --all-rust
Change-Id: Ic278ca9ec8fc48e779741f43b1fd53702b54396f
diff --git a/keystore2/tests/keystore2_client_list_entries_tests.rs b/keystore2/tests/keystore2_client_list_entries_tests.rs
index 1c0c496..8b3f700 100644
--- a/keystore2/tests/keystore2_client_list_entries_tests.rs
+++ b/keystore2/tests/keystore2_client_list_entries_tests.rs
@@ -140,7 +140,7 @@
                 let key_descriptors = keystore2.listEntries(Domain::APP, -1).unwrap();
                 assert_eq!(1, key_descriptors.len());
 
-                let key = key_descriptors.get(0).unwrap();
+                let key = key_descriptors.first().unwrap();
                 assert_eq!(key.alias, Some(alias));
                 assert_eq!(key.nspace, GRANTEE_UID.try_into().unwrap());
                 assert_eq!(key.domain, Domain::APP);