Use PollingCheck to handle random delay of focus status

1. The update of the focus status of an activity may be delayed
   after the activity is resumed. Use PollingCheck to accommondate
   the delay.
2. Passed the 100-iterations test.

Bug: 235351167
Test: atest
CtsCarBuiltinApiTestCases: ActivityManagerHelperTest#testSetFocusedRootTask
Change-Id: I591f57e3322a2e49ab7572e5429ea1ed0abc318e
1 file changed