Refactor RecentTasksList ctor to be testable

Made ctor accept more arguments so we can
pass in mock objects for unit testing.

Test: Created RecentsTasksListTest.java, passes
adb shell am instrument -w -r -e debug false -e \
class 'com.android.quickstep.RecentTasksListTest' \
com.google.android.apps.nexuslauncher.tests/androidx.test.runner.AndroidJUnitRunner

Fixes: 135687618
Change-Id: Iffda661e9d5329a2a403060bde674ab81e4e720e