[ Merge of http://go/wvgerrit/48640 ]
Usage information is saved periodically, in order to avoid excessive
flash writes. This limits our session usage accuracy to
within a usage save period. Saving usage information when
closing a session is an improvement and addresses some failures
seen with Netflix compliance tests.
Bug: 74015553
Test: WV unit/integration tests
Change-Id: I680aad05922f334df0611ff3933082a512f7c002