Simplify crash conditions in NetworkStackClient

The previous model could have impact on boot time to read/write from/to
disk, and could potentially fail in some scenarios where the device does
full reboots instead of framework restarts.

The current design most simply avoids crashing in the first 30mins after
a full reboot, and optimistically checks the wall clock to rate-limit
the crashes to every 6h.

Test: manual as below, without IS_DEBUGGABLE condition
Test: Install new NetworkStack, force crash, observe rollback
Test: Set min_uptime_before_crash to 100, force crash, observe crash
Test: min_uptime_before_crash still 100, install new NetworkStack, force
      crash: observe rollback (there was already a recent crash)
Test: Set min_crash_interval to 10, force crash: observe crash
Bug: 133725814
Change-Id: I3fd5ba7047d7ac991cb62a7cab16a40f4ee731a3