Improve undo support for text entered with IME

Use span properties to detect:
* Composing text - don't record undo operations
* Completing a composition - record an insert undo operation
* Canceling a composition - don't record

Save the composition state on parcel/unparcel.

Stop using begin/end batch edit to try to detect when a TextWatcher
is modifying the text. IMEs trigger multiple InputFilter passes in
a single batch edit. Use SpannableStringBuilder to determine when
we're in a TextWatcher callback because it is the authority on that
state.

Fix a bug in undo manager where it doesn't forget undos correctly if
there are more than one in the stack.

Bug: 19332904
Change-Id: Iaa9b0b2a7bf6683302cc85e7616e5d5fcc9fa202