Use same timestamp for on-disk BT Snoop Log and in-memory BT Snooz Log

* Logcat uses gettimeofday for its timestamp, the same as on-disk BT
  Snoop log
* Although in-memory BT Snooz Log uses the same method to get time, it
  is calling it separately, resulting in mismatch between timestamps of
  two snoop logs
* This CL let them uses the same timestamp_us value and put the function
  definition to libosi
* Note that preserved on-disk BT Snoop logs timestamp postfix at
      btsnoop_hci_<timestamp>.log
  will be changed to microsecond since epoch at current device timezone
  instead of the shifted BT Snoop timestamp value
* New unit tests for gettimeofday

Bug: 35113514
Test: Make, unit tests, run BT activities and check both snoop logs
Change-Id: I5b3f87bc523b272ced2c69a4595d0e0cbe29bcb3