ALLJOYN-1685 - Support android-x86 platform

a. New common configuration - conf/android/SConscript
   Checking for android-specific flags, ndk version and
   common compiler and linker flags.
b. Update the arm configuration - conf/android/arm/SConscript
   invoke the common configuration and then set arm-specific
   include & lib paths and compiler & linker flags.
c. Add the x86 configuration - conf/android/x86/SConscript
   Set the compiler, include & lib paths and compiler & linker flags
   from the NDK.

Change-Id: I28b8553c3e286afed55455d8a99d1c8248ceb104