This reverts 5898817 "Implement letterSpacing on Android >= 5.0".
Testing shows that that commit is the cause of facebook#19126, where in a
controlled TextInput after some text is first added, then deleted,
further interaction with the TextInput becomes extremely slow.

Fixes facebook#19126.

If we can, we'd like to get the benefit of the original change at
least on ordinary Text components.  The issue we're fixing seems like
it can't trigger there, or at least is much less likely to than on
TextInput.

So, try un-reverting all the changes to files that aren't specifically
about TextInput.

Unfortunately, this causes the issue to reproduce again.

In order to try to nail down what causes TextInputs to suffer this
issue even though all the specifically TextInput-related changes from
the original commit have been reverted, try knocking out one of the
places where the original commit causes its new code to get called.

The bug still repros, so this isn't enough.

Undo the last commit's experiment, and re-revert instead the other
spot where the original commit caused its new code to get called:
inside `buildSpannedFromShadowNode`.

This causes the issue to be fixed again!  So, this hunk is the
difference between TextInputs getting (painfully) slow and not.

Unfortunately, it also looks to me like it's probably the key entry
point to the whole change -- so keeping this knocked out has much the
same effect in practice as just reverting the entire original commit.