releasetools: Use int value of FIXED_FILE_TIMESTAMP.

Otherwise it gives a floating-point value (i.e. 1230768000.0). e2fsdroid
(i.e. the user of the value) didn't complain though, because it calls
strtoul(3) without checking endptr.

Bug: 80600931
Test: `m dist` with aosp_taimen-userdebug, check the build log.
Change-Id: I7467a60c947134b42f079c7a57fb8a9a9d4277da