Use find in Defaults::Value() to check for presence

`at` throws an exception when the value is missing. `at` is not
recommended. https://abseil.io/tips/224 has similar advice on
`std::vector::at`, go/totw/132#safe-lookup recommends a specific
alternative for maps.

This fixes the error
```
libc++abi: terminating due to uncaught exception of type std::out_of_range: absl::btree_map::at
```
Full invocation:
```
$ bazel run //cuttlefish/package:cvd -- fetch --default_build=git_main/aosp_cf_x86_64_only_phone-trunk_staging-userdebug --target_directory=$HOME/dl
$ bazel run //cuttlefish/package:cvd -- create --host_path=$HOME/dl --product_path=$HOME/dl --host_substitutions=bin/assemble_cvd,bin/run_cvd
...
SetFlagDefaultsFromConfig: No flag defaults to override.
libc++abi: terminating due to uncaught exception of type std::out_of_range: absl::btree_map::at
11-18 16:15:21.590 3669390 3669390 E cvd_internal_start: subprocess.cpp:207 Subprocess 3669416 was interrupted by a signal 'Aborted' (6)
11-18 16:15:21.591 3669390 3669390 E cvd_internal_start: main.cc:296 assemble_cvd returned -1
11-18 16:15:21.591 3667884 3667884 I cvd     : start.cpp:547 Device launch failed, cleaning up
```

Bug: b/461899525

Author: Databean

base/cvd/cuttlefish/host/libs/config/defaults/defaults.cc

@@ -19,6 +19,7 @@
 #include <string_view>
 
 #include <android-base/file.h>
+#include "absl/container/btree_map.h"
 
 #include "cuttlefish/host/libs/config/defaults/defaults.h"
 #include "cuttlefish/common/libs/key_equals_value/key_equals_value.h"
@@ -33,7 +34,13 @@ Defaults::Defaults(std::map<std::string, std::string> defaults) {
 }
 
 std::optional<std::string_view> Defaults::Value(std::string_view k) const {
-  return defaults_.at(k);
+  absl::btree_map<std::string, std::string>::const_iterator it =
+      defaults_.find(k);
+  if (it == defaults_.end()) {
+    return {};
+  } else {
+    return it->second;
+  }
 }
 
 std::optional<bool> Defaults::BoolValue(std::string_view k) const {