SQLite app ops schema: make device Id nullable

Almost all app op entries are supposed to be
for default device in the database, storing
the string for default device would take
~10 bytes for each row. Null are stored
very efficiently in SQLite db, so it will
reduce the disk space.

A test done with inserting 100k & 300k records
has shown 10% reduction in database size.

Bug: 377584611
Test: presubmit
Flag: android.permission.flags.enable_all_sqlite_appops_accesses
Change-Id: I5075d0137f5c5132b1c675d9499049bfb357f210