App-pairs baseline commit

- AppPairs: Main interface used to pair and unpair tasks.
- AppPairsController: Main controller for app-pairs and implements
AppPairs interface.
- AppPair: Manages a singal instance of an app-pair consisting of a
root task and 2 other tasks that are paired together.
- AppPairPool: Helper object pool for AppPair class so we don't have to
wait for a root task creation from system_server when we need an
app-pair.
- adb shell commands for pairing and unpairing tasks

Test: adb shell commands for pair/unpair
      adb shell dumpsys activity service SystemUIService WMShell \
        pair taskId1 taskId2
      adb shell dumpsys activity service SystemUIService WMShell \
        unpair taskId
Test: AppPairTests, AppPairsPoolTests, AppPairControllerTests
Bug: 172704672
Change-Id: Ie0a70c1aa764ad83b763931501234c1960251ca4