Correct a db perf test

The two multi-threaded read/write performance tests violate the
recommended Android database best practice: the writer loop creates a
new SQLiteDatabase on the test database file.  This leads to two
errors: the writer loop creates its database with an incompatible
journal mode in different from the actual database, and the test exits
without properly closing the database.

This change simply has the writer loop use the existing database for
its test.  The benchmark values for the multi-threaded read/write
tests change dramatically for the TRUNCATE journal mode and slightly
for the WAL journal mode; these metric changes are WAI.

Test: atest
 * SQLiteDatabasePerfTest

Bug: 318742652
Change-Id: Icb2534c6fbaf8daa3eb38e8b1b7f71331d571e9d