Fix BstatsCpuTimesValidationTest

The root cause of flakiness turned out to be the delay between
pressing the POWER button and the screen actually changing its
state to DOZE.  That multi-seconds delay would make batterystats
attribute the test app activity to screen-on, when in practice
the screen has already been turned off.

Bug: 363965015
Test: atest PowerStatsTest
Flag: EXEMPT_testonly

Change-Id: I644df36fa8cd2688514fb04a80c42de6cd8f7caa