Add feature flag for concurrent multi-session IME

Add `concurrent_input_methods` feature flag for concurrent
multi-session IME.

This CL also adds the `android.software.concurrent_input_methods` (
FEATURE_CONCURRENT_INPUT_METHODS) PackageManager feature to indicate
whether the device supports concurrent multi IME sessions.

Bug: 284527000
Test: m
Change-Id: I280d19c0dec12084fa49b90431a56e1b602def1c