android-demos: 增加String.replace()测试
#	modified:   android-demos/AndroidManifest.xml
#	new file:   android-demos/res/layout/layout_string_activity.xml
#	modified:   android-demos/src/com/example/android_demos/AndroidDemos.java
#	new file:   android-demos/src/com/example/android_demos/StringReplaceTestActivity.java
#