Do not actually reboot the device in VTS test.

- Sometimes reboot takes longer time and makes following tests fail.
- If the device has a screen lock, tests get stuck there.

This is not a new issue, but a recently added test (#launchApp) tries
to start an Activity and it fails when the device is still booting or at
a screen lock.

Bug: 185483743
Bug: 185576411
Test: atest FrameworksServicesTests:UpdatableFontDirTest
Test: atest UpdatableSystemFontTest
Change-Id: I86d4be1761e648114503b5f3560992a89c0b34d7