Fix SystemUI crash from back navigation trigger acquire wakelock.

PowerManager throw permission exception because core didn't clear
calling identitiy when calling startBackNavigation.
OTH, setLaunchBehind shouldn't pause current resume activity, will
be addressed separately from this CL.

Bug: 300428312
Test: launch two activity at once, to make the first activity stay in
initialized stage. Then when trigger gesture back, SystemUI won't crash
from receive SecurityException.

Change-Id: I5b283d631f151925cdc8877677df433660a5047c