Fix NPE in isTrimmable

Previous CL[1] makes the root home task nullable, which causes NPE
in RecentTask#isTrimmable. This CL adds null checks on the method and
other callers which don't have checks, and also marks
TaskDisplayArea#getRootHomeTask as @Nullable.

[1]: 3388bd2a

Test: atest RecentTasks RecentsAnimationControllerTest
Test: atest TaskDisplayAreaTests WindowStateTests
fixes: 155100679
Change-Id: I8a59fbcb99810172a3b3192917d24e5122a02476