Make ProcessStat collection more robust.

- Handle the case where start time is 0.
- Handle the case where getting uptime failed.
- Use 64bit int. (32bit is enough to record a time less than 24 days, but
  just in case.)

Bug: 315061143
Test: m test-art-host-gtest-art_runtime_tests
(cherry picked from https://android-review.googlesource.com/q/commit:d358b579a98f8dcc5e1ba6678649543df51f7af5)
Merged-In: I3ac546f6921b8d6add2491170b5d8f50cc7169c1
Change-Id: I3ac546f6921b8d6add2491170b5d8f50cc7169c1