Use session with longest remaining duration when session sharing is used.

[ Merge of https://go/wvgerrit/16940 ]

An alternate scenario to renewing keys is to load the same keys in
a separate session and make use of them by using the session sharing
feature.

Session sharing involves iterating through a map of sessions and
returning the first session that contains the Key ID. In certain cases
(license about to expire) we might prefer an alternate session
be chosen.

Licenses may expire in two ways. Policy engine, driven by a 1 second
timer may detect expiry and send an asynchronous event. OEMCrypto may
also detect expiry based on information in the key control block
and return an error during decryption. It is possible that these
may differ by upto a second. This can lead to issues where decryption
fails but EVENT_KEY_EXPIRED is not generated till later.

It is possible to address this by using information from both timers
to notify the app about expiry. To implement this correctly will
add complexity and require synchronization between threads. To avoid
this an alternate solution is, if session sharing is used, to pick
the session that has a license with the longest remaining validity.

b/27041140

Change-Id: I398cc4c10ee3a2f192d4a0befe7c8a469dd5bf86

libwvdrmengine/cdm/core/include/policy_engine.h

@@ -84,6 +84,7 @@ class PolicyEngine {
   bool IsPlaybackStarted() { return playback_start_time_ > 0; }
 
   bool IsLicenseOrPlaybackDurationExpired(int64_t current_time);
+  int64_t GetLicenseOrPlaybackDurationRemaining();
 
   bool CanRenew() { return policy_.can_renew(); }