Add custom scalar/aggregate functions to SQLite.

SQLite ships with a handful of basic functions, such as UPPER() as
a scalar function and MAX() as a aggregate function.  We now have
several use-cases for adding custom functions, where it's otherwise
prohibitively expensive to perform post-processing on the returned
Cursor, as that requires copying processed data to yet another
MatrixCursor before returning to apps.

This change adds the ability for developers to register custom
scalar and aggregate functions on databases that they've opened;
some contrived examples are scalar functions like REVERSE() for
reversing a string, or aggregate functions like STDDEV().

To give developers the most flexibility, we use the Java functional
interfaces for defining these operations, as developers may already
be familiar with the contracts of those methods.  This also opens
the door to quickly adapting existing code through utility methods
like BinaryOperator.minBy(Comparator).

Bug: 142564473
Test: atest CtsDatabaseTestCases:android.database.sqlite.cts.SQLiteDatabaseTest
Change-Id: I9fa0e60ec77bab676396729cc9cb8ba8aaf56224