Clear app token mAppStopped when app resumes.

It is possible for an activity to be in the stopped state without
setting it's visiblility to false in window manager.
For example, the home acitivty behind the lock screen. Since the
lock screen isn't an activity it doesn't affect the visiblity set
of the home activity, so AM doesn't tell WM to hide the app token.
However, AM uses another channel to detect that the device is locked
and moves the activity into stopped state. WM on the other hand also
detects that the device is locked and hides the window surfaces of
all windows behind the lock screen. So, at this point AM has also
told WM that the activity is stopped. Once you unlock the screen
AM resumes the activity but doesn't report any visiblility changes to WM
since it's internal state didn't change. So, if you go from the home
activity to another app the home activity window will be destroyed
before the activity is stopped because mAppStopped is set to true.
We now set mAppStopped to false when the activity is resumed.

Bug: 27286867
Change-Id: Ic75456d30abd582fa44f932f5aeeb449950157ee