[RESTRICT AUTOMERGE]: Clean up SecurityTestCase.java

* cleaned up imports
* refactored to prevent conflicts
* removed duplicated code for getting device uptime
* made the "hard reset" failure code readable
* removed an unnecessary wait for device available
	the check was crude and immediately preceded by a proper check

Test: cts-tradefed run cts -m CtsSecurityBulletinHostTestCases
	make sure to reboot device mid-run and expect a "hard reset"
Bug: 128697496

Change-Id: I416e551c52ae5d1259c19210cba7a1c96cea78db