Add License::Policy::play_start_grace_period_seconds

[ Merge of http://go/wvgerrit/22565 ]

When using the grace period, the CDM will need to override the values
given to use by the TEE (through OEMCrypto).  Normally the first (and
last) decrypt times are stored securely by the TEE.  To avoid extra
complexity in OEMCrypto, we will simply ignore the values given to us
by the TEE when using this feature.

However, the TEE will still enforce the (hard) license duration.  So
only the rental/playback durations will be affected by malicious
editing of files.

b/34211676

Test: Reran unittests including newly added tests. All tests other than
some oemcrypto, request_license_test passed. Those tests failed with
or without this CL.

Change-Id: I6d7b5bfb669fd8603b474b68c2f7175b0c30901d

libwvdrmengine/cdm/core/src/device_files.proto

@@ -39,6 +39,10 @@ message License {
   optional int64 playback_start_time = 8 [default = 0];
   optional int64 last_playback_time = 9 [default = 0];
   repeated NameValue app_parameters = 10;
+  // This will be 0/missing if the grace period has not expired; otherwise it
+  // contains the playback_start_time we should use as an override.  This is
+  // ignored if there is no grace period.
+  optional int64 grace_period_end_time = 11 [default = 0];
 }
 
 message UsageInfo {