Fix for zero-sized corrupted license files

Netflix reported that after pulling power while their app is active,
the app isn't able to restart.  This is because the license file for
session keys isn't getting synched to disk, so the data is still in
the buffer cache when the device shuts down.  Calling fflush and fsync
on the file ensures the data is persisted to disk.  fclose alone
doesn't do fsync.

In testing, I also noticed that the license file was being rewritten
every second which is hard on the flash filesystem.  The timer thread
was modified to avoid these frequent writes.

Merge of https://widevine-internal-review.googlesource.com/#/c/12431/
from the widevine cdm repo.

bug: 19108207
Change-Id: Ibe81e40a3c1f5d25563523da43fefdccdaa6ddcf

libwvdrmengine/cdm/core/src/cdm_session.cpp

@@ -568,13 +568,12 @@ void CdmSession::OnTimerEvent(bool update_usage) {
   if (update_usage && has_decrypted_recently_) {
     policy_engine_->DecryptionEvent();
     has_decrypted_recently_ = false;
+    if (is_offline_ && !is_release_) {
+      StoreLicense(DeviceFiles::kLicenseStateActive);
+    }
   }
   policy_engine_->OnTimerEvent(&event_occurred, &event);
 
-  if (is_offline_ && !is_release_) {
-    StoreLicense(DeviceFiles::kLicenseStateActive);
-  }
-
   if (event_occurred) {
     for (CdmEventListenerIter iter = listeners_.begin();
          iter != listeners_.end(); ++iter) {

libwvdrmengine/cdm/src/file_store.cpp

@@ -71,7 +71,9 @@ bool File::Open(const std::string& name, int flags) {
 }
 
 void File::Close() {
-  if (impl_->file_) {
+  if (impl_->file_){
+    fflush(impl_->file_);
+    fsync(fileno(impl_->file_));
     fclose(impl_->file_);
     impl_->file_ = NULL;
   }