aaudio: use unique_ptr in FixedBlockAdapter

Code cleanup.

Bug: 151650670
Test: atest CtsNativeMediaAAudioTestCases
Change-Id: I980862379ade9b6cc62f1743c916208e3911a00a
Merged-In: I980862379ade9b6cc62f1743c916208e3911a00a
(cherry picked from commit 2f39c0b0fcbc5b857df2823247d163f2ba1c14a8)
4 files changed