Reduce unnecessary invocation of pokeDrawLock

Now it only acquires wake lock (poke draw) for focused activity
window. Because the original purpose (commit c2932a1b) is to make
sure that ambiactive activity can draw without being suspended
for wearable devices.

Check focus instead of visibility because pokeDrawLockIfNeeded
can be called from DisplayListener which happens before the
visibility change.

On a regular phone device with AOD enabled, this can reduce the
number of invocation:
 30+ times when turning screen off.
 4 times per minute (the clock on AOD).
i.e. pokeDrawLock will never be called on a device with keyguard.

Bug: 218406154
Test: Enable AOD, turn off screen to enter AOD.
      Check the section "Wake Lock Log" of "adb shell dumpsys power".
      There is no more "ACQ Window:xxx (draw)".
      And after several minutes, the clock on AOD still shows the
      latest time (because com.android.systemui.doze.DozeUi#onTimeTick
      also acquires WakeLock).
Change-Id: I6fc7f6afec68c63124d76c4734285188ab62fc41