Enable shadow call stack in packages/apps/Bluetooth.

Shadow call stack (SCS) is a security mitigation that uses a separate
stack (the SCS) for return addresses.

This change causes libbluetooth_jni.so to be built with SCS.

For the moment this is an internal-only change until the AOSP kernels
can be fixed to not clobber x18.

Bug: 112907825
Bug: 120630739
Change-Id: I3b0ac803e3494ae919f06cf1c04fc644db7eabdd