Restore activityPaused to a sync binder call

The intention of previous one-way change is to reduce blocking
call on client side. But in most of cases the blocking of
activityPaused is no harm for the app. Because either the app
is going to background or it launches another activity in the
same process. If the case is the latter, the creation of next
activity is still scheduled after activityPaused is done.

Some apps may even use Idler to start some post initialization.
They assume the timing of idle will happen after the whole
consecutive launch in the same process. Then the non-blocking
call may break their assumption that the idle event is triggered
earlier.

Bug: 161781274
Bug: 178005687
Test: CtsWindowManagerDeviceTestCases
Test: In the onCreate of first activity, start the second activity
      and set an Idler on UI thread. The Idler should be called
      after the second activity is launched.

Change-Id: Ib256d0a534fedb0b39f2825f4deb2f8c81e857be