TIMOB-13392: Android scrollable tabs

Android tab group can now by default be navigated by swipes (e.g. see Play Store app). The functionality can be toggled with the new `TabGroup.swipeable` property. In addition, tab navigation can be disabled (or re-enabled) by calling `disableTabNavigation(boolean)`. Plus added tab `selected` and `unselected` events (currently these events exist in Android only. In iOS the tab focus/blur events do the same - I suggest they be renamed to selected/unselected). Note that all additional events are fired exactly as before.
@pingwang2011 this is internal to the implementation and not accessed from Javascript. In TabGroupProxy.java it is indeed a getter/setter - here it is of no consequence in my opinion.