TargetAudioEncTest: use vector instead of malloc

The static analyzer noted that we'll fail to `free` this `malloc()` if any
ASSERT between it and the `free()` fails. Just use a `vector<char>`
instead, since that handles freeing for us.

Bug: 206470603
Test: TreeHugger
Change-Id: I2c7edab803420beb9f4d72c83f6a770fb053413e