Measure baseline IMF latency (1/n)

Measure latency caused by IMF for IME operations like show / hide.
This CL uses apct tests for integration with Crystallball to measure
overall latency and breakdown of critical IMF methods.
In this CL we introduce a BaselineIme with minimal UI to measure
user-preceived delays in IME show/hide.
Refer to design doc in bug.

Bug: 167947940
Test: atest ImePerfTests and also refer to README.md
Change-Id: I8efff52fe25952d452aef7f059400c63d1a9fa4a