[flexiglass] Hydrates system server with isDeviceLocked state.

See associated bug for details of debugging and implementation design.

Fix: 343211758
Test: unit tests added
Test: manually verified with adb shell dumpsys trust | grep deviceLocked
that the deviceLocked state is =1 when locked and =0 when unlocked
Test: manually verified with adb shell setprop
log.tag.TrustManagerService VERBOSE && adb shell stop && adb shell start
&& adb logcat -s TrustManagerService | grep handler that the order of
received message IDs matches with flexiglass on and off when unlocking
or locking
Test: manually verified that the overview screenshots are shown, see
b/347789654 for more information
Flag: com.android.systemui.scene_container

Change-Id: I07449eacbb4c3cc053f4f27483894e5305d4f3b6