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.

(cherry picked from commit fe16b5cc)

Change-Id: Id69681a35c7eb2f0eb21b48e3616dcda82ce41b8