FragmentTransaction.commitNow, framework edition

Offer commitNow and commitNowAllowingStateLoss methods on Fragment for
use by encapsulated components using fragments as implementation
details. This can help prevent unexpected ordering side effects at the
app level when a call to a library method wants to commit and
immediately initialize a fragment as an implementation detail.

Note that this change still does not permit reentrant FragmentManager
operations. It is still an error to add/remove/change fragments in the
same FragmentManager while a fragment transaction is being executed.

Have the commonly used ViewPager adapters use commitNow instead of
executePendingTransactions.

Change-Id: Ia37a871234a287423063f0c2c3e4c93d69116cad
(cherry picked from commit f6b30662)