Split up locktask reboot CTS

Split up DeviceOwnerTest#testLockTaskAfterReboot_deviceOwnerUser into
two tests. This prevents the problem reported in http://b/111383175
where the test runner was moving onto the next step before finishing
LockTaskHostDrivenTest#testLockTaskisActiveAndCantBeInterrupted.

Also adds javadoc to that method explaining the problem.

Test: atest DevivceOwnerTest
Bug: 111383175
Change-Id: I19e2f139fffcc6c5ab5d2a8a30561ef96d7ab47c
2 files changed