NativeMIDI: Fix access to unaligned 64-bit data

Accessing unaligned 64-bit data in 32-bit ARM code leads
to SIGBUS / BUS_ADRALN fault. Since the timestamp position
in the message buffer is arbitrary, use memcpy to copy
it to the output argument.

Bug: 114123427
Test: 32-bit android.nativemidi.cts.NativeMidiEchoTest
Change-Id: I0cb08cb0b124e3f4a05ea19bf91f40081a584a2e