aconfig: Split FeatureFlagsImpl templates.

The FeatureFlagsImpl.java template was getting pretty long and
difficult for me to read, so split into separate files (one for internal
mode new storage, one for new exported mode new storage, and one for
test mode). Left the original FeatureflagsImpl.java template alone
rather than split it further or remove dead code because we will delete
it soon anyway. We just need to handle force RO mode before deleting
that file.

manual testing:
cargo run --create-cache --package com.android.aconfig.test --container
system --cache tmp --declarations tests/test.aconfig --values
tests/first.values
cargo run -- create-java-lib --cache tmp --out .
then adding a bunch of settings (--mode exported, --mode
force-read-only, --mode test, --allow-instrumentation true,
--new-exported true) with both this change and without this change and
ran a diff on the generated files. only difference was whitespace

Test: atest aconfig.test, manual
Change-Id: I2add6341995130377c967b894be62690cf5704b7