libsnapshot: Set compile_multilib to "first"

vts_libsnapshot_test32 fails as library android.hardware.boot@1.0.so
isn't built for 32-bit.

This will still test on 32-bit platforms.

Bug: 388929904
Test: vts_libsnapshot_test
Change-Id: I16d37095f3057b12c101a80e1493fa659ead3e3b
Signed-off-by: Akilesh Kailash <akailash@google.com>