Add showSoftKeyboard(boolean show) to JmeSystem.
Includes empty implementations for now.  Android to be implemented in next commit.
Implement showSoftKeyboard for Android.
Also changed JmeAndroidSystem to store the view instead of the activity.
When users call JmeSystem.showSoftKeyboard(true), the default Android soft keyboard is displayed and any key presses are sent to jME as touch events.  Key events are also generated so the user can use the same key listeners as Desktop to keep the user code common between Desktop and Android platforms.