aaudio: align AAudioServiceMessage for 32/64 bits

Chromebook sometimes runs 32 and 64-bit code together.
That caused messages between the code to be rejected because the
struct sizes did not match.
So we used alignas(8) to force members to 8 byte boundaries.

Bug: 155901581
Test: Launch OboeTester
Test: Tap TEST OUTPUT
Test: Tap OPEN and START
Test: You should get "MMAP YES" if MMAP is supported by the device.
Change-Id: I0cd6a59a974888ef17b06b38fa78f032d602cd57