Add notifyInitialized(), notifyReleased() & isInitialized() methods in TaskView.

- These will be helpful in carrying out initialization and clean-up logic in sub-classes.

Bug: 235151420
Test: atest TaskViewTest
Change-Id: I5d357ff742597f3b02150364404e3f3a7c2d49b0
(cherry picked from commit a72e61eb)