Don't throw exception for duplicate stop

A double stop request is possible when display is being locked.
An activity may receive a "sleep" message followed by "stop",
both of which try to move it to stopped state.

An example when this happens: a keyguard is set up and the screen
is being locked. The keyguard will occlude the activity, which
causes a transition to paused state and, eventually, to stopped
state. A "sleep" message can be sent sometime before "stop" message
and will ignore that activity is in the process of becoming stopped.

Change-Id: I09e2c26004664b6e73ac5c2b6fe88bdf8271cf34
Fixes: 74967786
Test: FrameworksCoreTests:ActivityThreadTest