Merge some unit test classes

Merge
TaskRecordTests into TaskTests.
TaskStackTests and ActivityStackTests into RootTaskTests.
RootActivityContainerTests into RootWindowContainerTests.
ActivityDisplayTests into DisplayContentTests.

Also move some TaskDisplayArea tests to TaskDisplayAreaTests.

Bug: 144248496
Test: atest TaskTests RootTaskTests
      atest RootWindowContainerTests
      atest DisplayContentTests
      atest TaskDisplayAreaTests
Change-Id: I4ef9abad9e73d5ecb1bc19beda890e10a478d172