Prevent native methods in InputManagerService from being removed

Add a proguard rule to prevent native methods in InputManagerService
from being removed. If proguard removes one of these methods, the system
crashes at boot when it tries to register the native implementation
through JNI, resulting in a hard-to-debug bootloop.

Bug: None
Test: manual
Change-Id: I528bd97c9ef179ff18e3e0ad95a4e9a015b7e6c9