Query BatteryManager for initial value

Test suites will crash systemui before beginning. Sometimes it
can take just a bit too long to get the battery update
broadcast, so query for it instead.

Test: atest KeyguardUpdateMonitorTest
Fixes: 375235694
Flag: EXEMPT bugfix
Change-Id: If2db43f9711539b23f10b9f6311d25d1526b6f97