android/benchmark: implement reboot as part of new _preInit
android/benchmark: implement reboot as part of new _preInit

When --boot-image is present we can trigger reboot from a newly
introduced private _preInit() callback (instead of letting the
benchmarks implement the rebooting scheme). This should improve the
design and make benchmarks implementations less error prone.

Signed-off-by: Juri Lelli <bdc6bcd4e8b34b4b92413d758024aeca6da0e459@arm.com>

arnoldlu/lisa,bjackman/lisa,ARM-software/lisa,credp/lisa,arnoldlu/lisa,credp/lisa,ARM-software/lisa,credp/lisa,mdigiorgio/lisa,mdigiorgio/lisa,bjackman/lisa,credp/lisa,ARM-software/lisa,ARM-software/lisa