Add support for appending standalone phrases.

When users are building queries, they often need to append several
standalone SQL clauses, and it's tedious to track their first clause
so they can manually append " AND " to each subsequent clause.

So add new appendWherePhrase() API which appends a standalone phrase
which is AND'ed together with any existing WHERE query.

Also fix bug in update() which would turn null values into the
string literal "null" instead of passing them through as SQL NULL.

Test: atest cts/tests/tests/database/src/android/database/sqlite/cts/SQLiteQueryBuilderTest.java
Bug: 111085900
Change-Id: Ia280dd864895654239503e080eaef925f5620d37