Add InputConnection#takeSnapshot()

This CL introduces InputConnection#takeSnapshot() so that the system
can take the text context snapshot at any time.

Note that this new API is not intended to be used by the IME for now.

See also the corresponding CTS CL [1] about how it works.

 [1]: Id94464ebaffd81b47c32d7ca3abc8c94c3485658

Fix: 203086234
Test: atest CtsInputMethodTestCases
Change-Id: I8e00dacce5ca915e276fdd8288ffc9167eb01a26