// "Current" indicates the discovery document was recently
  // refreshed. "Stale" indicates the discovery document could not
  // be retrieved and the returned discovery document may be
  // significantly out of date. Clients that require the latest
  // version of the discovery information be retrieved before
  // performing an operation should not use the aggregated document
  optional string freshness = 3;

These are different things.
The Android cache is internal to certain tasks in the Android plugin, and will eventually be removed in favor of native Gradle support.

== Why use the build cache?

The build cache can _significantly_ improve build performance for Android projects, in many cases by 30-40%.
Many of the compilation and assembly tasks provided by the Android Gradle Plugin are cacheable, and more are made so with each new iteration.

  // "Current" indicates the discovery document was recently
  // refreshed. "Stale" indicates the discovery document could not
  // be retrieved and the returned discovery document may be
  // significantly out of date. Clients that require the latest
  // version of the discovery information be retrieved before
  // performing an operation should not use the aggregated document
  optional string freshness = 3;

 *
 * <ul>
 *   <li>If you only access one end of the queue, and do use a maximum size, this class will perform
 *       significantly worse than a {@code PriorityQueue} with manual eviction above the maximum
 *       size. In many cases {@link Ordering#leastOf} may work for your use case with significantly
 *       improved (and asymptotically superior) performance.

	// "Current" indicates the discovery document was recently
	// refreshed. "Stale" indicates the discovery document could not
	// be retrieved and the returned discovery document may be
	// significantly out of date. Clients that require the latest
	// version of the discovery information be retrieved before
	// performing an operation should not use the aggregated document

 * good job of distributing the elements to the buckets to a distribution not far from uniform), and
 * amortized since some operations can trigger a hash table resize.
 *
 * <p>This implementation consumes significantly less memory than {@code java.util.LinkedHashSet} or
 * even {@code java.util.HashSet}, and places considerably less load on the garbage collector. Like

 * good job of distributing the elements to the buckets to a distribution not far from uniform), and
 * amortized since some operations can trigger a hash table resize.
 *
 * <p>This implementation consumes significantly less memory than {@code java.util.LinkedHashSet} or
 * even {@code java.util.HashSet}, and places considerably less load on the garbage collector. Like

	// interface type, allowing us to detect any (even with the legacy
	// representation), and format it as "any" rather than interface{}, which
	// clarifies user-facing error messages significantly.
	//
	// TODO(rfindley): once the gotypesalias GODEBUG variable is obsolete (and we
	// consistently use the Alias node), we should be able to clarify user facing

	// interface type, allowing us to detect any (even with the legacy
	// representation), and format it as "any" rather than interface{}, which
	// clarifies user-facing error messages significantly.
	//
	// TODO(rfindley): once the gotypesalias GODEBUG variable is obsolete (and we
	// consistently use the Alias node), we should be able to clarify user facing

 * from uniform), and amortized since some operations can trigger a hash table resize.
 *
 * <p>As compared with {@link java.util.LinkedHashMap}, this structure places significantly reduced
 * load on the garbage collector by only using a constant number of internal objects.
 *
 * <p>This class should not be assumed to be universally superior to {@code