Fix compiling bug in android_armv7_demo : cxx demo can not compile properly because demo' android version is different from that of library file (#2308)

link armv7 demo to android-23 to make sure the demo can compile properly
cxx demo can not compile properly because demo' android version is different from that of library file, we fix this bug by linking armv7 demo to android-23

Add 'inference' folder into build.lite.x86, 'inference' folder contains head files、library files and demo of x86 compiling (#2301)

Add inference folder into build.lite.x86, build.lite.x86/inference folder contains head files、library files and demo of x86 compiling

* [framework][place] remove prefered_place, use place order in valid_place array instead test=develop

* remove kHost from valid_places test=develop

* Add precision and persistable attrs for the tensor. And fix cxx light and full api demo.

* update precision2string methods. test=develop

* move the save logic to the front of the run in mobilenetv1_full_api.cc, test=develop.

* add comments for UpdateVarsOfProgram. test=develop