fix(environment): cache App Store environment checks

[paulb777]

Summary

Cache +[GULAppEnvironmentUtil isAppStoreReceiptSandbox] and
+[GULAppEnvironmentUtil isFromAppStore] for the lifetime of the process.

Part of firebase/firebase-ios-sdk#16726.

Why

firebase/firebase-ios-sdk#16726 reports main-thread hangs in production, some
ending in watchdog terminations, on two Firebase paths that both end in
-[NSBundle appStoreReceiptURL]. That property is not a file lookup. It calls
+[LSBundleProxy bundleProxyForCurrentProcess], which ends in
xpc_connection_send_message_with_reply_sync, so the caller waits for the
LaunchServices daemon. That can take seconds when the daemon is busy.

Before this change, isAppStoreReceiptSandbox had no cache. isFromAppStore
cached only IsAppEncrypted(). Every call from every caller (GULLogger,
FirebaseCore's user agent, Messaging, Auth, Performance) repeated the XPC round
trip. All inputs (receipt type, bundle contents, Mach-O encryption info, and the
FirebaseAppStoreReceiptURLCheckEnabled Info.plist key) are fixed for the life
of the process.

Changes

• The existing method bodies move, unchanged and with their comments, into
  file-local IsFromAppStore() and IsAppStoreReceiptSandbox() helpers. The
  public class methods wrap them in dispatch_once.
• IsFromAppStore() still calls the now-cached +isAppStoreReceiptSandbox, so
  the receipt URL is read at most once per process across both APIs.
• Removed the now-redundant inner dispatch_once around IsAppEncrypted().

Notes for reviewers

• Caching makes the cost once per process. It does not move that first call
  off the calling thread. The Firebase PRs move the callers off the main
  thread (links below).
• dispatch_once callers that arrive while the first computation is running
  wait for it to finish. Previously they would each have made their own XPC
  call.

Testing

• New testAppStoreReceiptURLIsReadOnlyOnce: after a warm-up call, it swizzles
  -[NSBundle appStoreReceiptURL] with a counter and asserts that neither API
  reads it again. A sanity check confirms the swizzle intercepts reads. The test
  fails on main (receiptURLReadCount is 2) and passes with this change.
• xcodebuild -scheme GoogleUtilities-Package -destination 'platform=macOS' test: all suites pass (160 UtilitiesUnit tests plus the Swift and ObjC
  import tests).

Related PRs

• fix(core): build heartbeat user agent off the main thread firebase/firebase-ios-sdk#16842: builds the heartbeat user agent off the main thread (Path 2).
• fix(messaging): reduce main-thread work when setting the APNs token firebase/firebase-ios-sdk#16843: reduces main-thread work when setting the APNs token (Path 1).