Merge "Add WriteHashTree() to allow users process hashtree data themselves" am: 247fc54ea6 am: 5492f04052 am: da9049b5a8

Original change: https://android-review.googlesource.com/c/platform/system/extras/+/1441311

Change-Id: I9efba6d885e396faccec83d8afe0ad7e8adda70d