Define LOCAL_PRIVATE_PLATFORM_APIS for bttestapp

For System SDK compliance either of two flags should be defined
for all Android APK's

LOCAL_SDK_VERSION Or LOCAL_PRIVATE_PLATFORM_APIS

LOCAL_SDK_VERSION supports two values current and system_current.
"current" will compile APK code with SDK API's and system_current
will compile code with SDK + System API's

LOCAL_PRIVATE_PLATFORM_APIS will compile APK code with platform
API's, this option enables to use hidden platform API's in APK.
This flag cannot be defined for vendor APK's.

Change-Id: I3c56335396dd6275c0eba64646a6980acdf0422f