Add the TextAttribute class and variant of setComposingText(),
setComposingRegion(), commitText() to support IME to take extra
information to editor.
Bug: 198732184
Test: atest CtsInputMethodTestCases:InputConnectionEndToEndTest CtsInputMethodTestCases:TextAttributeTest
Change-Id: Ic9c917b70726ca32c2097b0374951c20af9624c4