save uncrypt status to last_install

Save the uncrypt time cost to /cache/recovery/uncrypt_status. Recovery
reads the file and saves its contents to last_install.

Bug: 31383361
Test: Tested on angler and uncrypt_time reports correctly.

Change-Id: I5cd3f7b6ca069d69086d09acfea8fc4f1215c833
Merged-In: I5cd3f7b6ca069d69086d09acfea8fc4f1215c833