Mark 4 tests as FlakyTest

Until b/113616538 is fixed, a test class without FlakyTest, can be included in
a presubmit test mapping suite unexpectedly. Mark these 3 classes as FlakyTest
for now:
ActivityManagerServiceTest
AssistDataRequesterTest
AppErrorDialogTest

This test becomes unstable and still needs to be fixed before it can run in
presubmit:
TaskPositioningControllerTests

Bug: 117880789
Test: none
Change-Id: I068d010fa328fa0ec36c8898e21cdf2da7abf0a2