Preserve x18 while calling aptX encoder libraries.

Shadow call stack (SCS) is a security mitigation that uses a
separate stack (the SCS) for return addresses. In versions of
Android newer than P, the compiler normally cooperates with the
system to ensure that the SCS address is always stored in register
x18. This requires that everything in a process that uses SCS is
built with -ffixed-x18. However, two libraries (libaptX_encoder.so
and libaptXHD_encoder.so) are supplied in binary format and were
not compiled with -ffixed-x18. We are working with the vendor to get
these libraries rebuilt with -ffixed-x18, but until then, we need to
prevent this library from clobbering x18 so that we can turn on SCS
in the Bluetooth process.

Bug: 112907825
Bug: 120621517
Change-Id: Id27829ed5696903c42b4aeb75c3b3880c97a3a36