Enable count-based expiration for media metrics

// (0 for either of these disables that threshold)

//

static constexpr nsecs_t kMaxRecordAgeNs =  28 * 3600 * (1000*1000*1000ll);

static constexpr int kMaxRecords    = 0;

// 2019/6: average daily per device is currently 375-ish;

// setting this to 2000 is large enough to catch most devices

// we'll lose some data on very very media-active devices, but only for

// the gms collection; statsd will have already covered those for us.

// This also retains enough information to help with bugreports

static constexpr int kMaxRecords    = 2000;

// max we expire in a single call, to constrain how long we hold the

// mutex, which also constrains how long a client might wait.