Removed the ModelTree class. Adaptive CPU simply has a unique pointer to tree root now. Adjusted tests.

Test: atest libadaptivecpu_test:TreeReaderTest & atest libadaptivecpu_test:ModelTreeTest
Bug: b/246317222.
Change-Id: Id961898e270a06a56c4f2a8a5578b44201cbe663
9 files changed