Add ability to make task snapshots store in RGB565

Bug: 123338864
Bug: 135150273
Test: run app-launch-perf test (go/platform-startup)
Test: launch apps manually and measure the execution time of
      snapshotTask(), writeBuffer(), and loadTask()
Change-Id: I9841c6dad99b1f4d4ba0044b7fe06fe021c295a0