Modified Google IMA samples to handle seeking to the previous position

when resuming ad playback.

AdRulesIMASampleApp/src/main/java/com/brightcove/player/samples/ima/adrules/MainActivity.java
BasicIMASampleApp/src/main/java/com/brightcove/player/samples/ima/basic/MainActivity.java
BasicIMAWidevineSampleApp/src/main/java/com/brightcove/player/samples/imawidevine/basic/MainActivity.java

  * Added AD_POSITION constant.

  * Added adPosition and adWasPlaying variables to hold the state.

  * Modified constructor to restore state.

  * Added overrides for onStart(), onPause(), onResume(),
    onSaveInstanceState(), and onStop() to capture and restore the ad
    position.