Introduce IME API for adjusting handwriting idle timeout

The API allows IME developers to override default system idle-timeout
for stylus handwriting sessions. The system default may be too long or
too short for certain use-cases and it makes sense to let IME change it.

Bug: 233811329
Test: atest StylusHandwritingTest
Change-Id: I9e869658ad57b2ec91b882ab9e003e7f60f95c7c