[Android] Add android aar package (#416)

* [Android] Add Android build docs and demo (#26)

* [Backend] Add override flag to lite backend

* [Docs] Add Android C++ SDK build docs

* [Doc] fix android_build_docs typos

* Update CMakeLists.txt

* Update android.md

* [Doc] Add PicoDet Android demo docs

* [Doc] Update PicoDet Andorid demo docs

* [Doc] Update PaddleClasModel Android demo docs

* [Doc] Update fastdeploy android jni docs

* [Doc] Update fastdeploy android jni usage docs

* [Android] init fastdeploy android jar package

* [Backend] support int8 option for lite backend

* [Model] add Backend::Lite to paddle model

* [Backend] use CopyFromCpu for lite backend.

* [Android] package jni srcs and java api into aar

* Update infer.cc

* Update infer.cc

* [Android] Update package build.gradle

* [Android] Update android app examples

* [Android] update android detection app

java/android/.gitignore

@@ -0,0 +1,20 @@
+.DS_Store
+.idea
+.gradle
+.cxx
+cache
+build
+app/cache
+app/libs/fastdeploy*
+app/.cxx
+app/build
+app/src/main/assets/models/*
+app/.gradle
+app/.idea
+fastdeploy/cache
+fastdeploy/libs/fastdeploy*
+fastdeploy/.cxx
+fastdeploy/build
+fastdeploy/src/main/assets/models/*
+fastdeploy/.gradle
+fastdeploy/.idea