monitor: Opencode strndupa in print_string

monitor: Opencode strndupa in print_string

Strndupa is GNU extension. print_string is simple and opencoding
strndupa is not a big loss.

This fix linkage error with bionic library which doesn't provide
strndupa.

target Executable: btmon (out/target/product/grouper/obj/EXECUTABLES/
    btmon_intermediates/LINKED/btmon)
prebuilts/gcc/linux-x86/arm/arm-linux-androideabi-4.6/bin/../lib/gcc/
    arm-linux-androideabi/4.6.x-google/../../../../
    arm-linux-androideabi/bin/ld: out/target/product/grouper/obj/
    EXECUTABLES/btmon_intermediates/monitor/sdp.o: in function
    print_string:external/bluetooth/bluez/monitor/sdp.c:146: error:
    undefined reference to 'strndupa'
collect2: ld returned 1 exit status
make: *** [out/target/product/grouper/obj/EXECUTABLES/
    btmon_intermediates/LINKED/btmon] Error