Add View#generateViewId; make RadioGroup use it

Bug 6448164

generateViewId provides a way for applications to generate opaque ID
values suitable for use with View#setId that will not collide with
values generated by aapt for R.id.

Fix a bug where RadioGroup assumes object hash codes will always be
positive.

Change-Id: I3e2870cd672d6061bb465128f428c81aeef0c44b