DO NOT MERGE - fix FTS3 column pointer handling

Add new interfaces sqlite3_result_pointer(), and sqlite3_value_pointer()
and use them to transfer the eponymous FTS3 column pointer to the
snippet() and offsets() routines.

Bug: 63852675
Test: presubmit
Change-Id: I0f0be2bfc21aef8917da63d76d1007fbf8c8ad4d