Add EditorInfo#getInitialSurroundingText API

Add EditorInfo#getInitialSurroundingText() to support retrieving
surrounding text arround cursor with one API, similar to
InputConnection#getSurroundingText().

Migreate EditorInfo#InitialSurroundingText class to use SurroundingText.

Create a copy of the surrounding text when creating EditorInfo instance.

Bug: 167947745
Bug: 122691437
Test: atest FrameworksCoreTests:EditorInfoTest
Test: atest CtsInputMethodTestCases:EditorInfoTest
Change-Id: I7fe1e585b75f7db5e57a14e4ac91cc9a3282f293