ndk_async_codec_fuzzer: Bug Fix

Resolved Abort due to improper thread handling. Implemented a thread pool to handle the lifecycle of all the threads in the fuzzer.

exec/s: 681
Test: ./ndk_async_codec_fuzzer clusterfuzz-testcase-minimized-ndk_async_codec_fuzzer-5126771558973440
Bug: 271261428

Change-Id: Icbd310c795fa76db8594d195b250e065ee2941fa
(cherry picked from commit 49b25217341a7b460c2336b6f7e4210bb3dce692)