Add startImeTrace and stopImeTrace APIs in ImeTracing

So Settings DevelopmentTiles doesn't need to create direct
IInputMethodManager dependency to access system API.

Bug: 175742251
Test: Manually test ime winscope works properly
Test: make RunSettingsRoboTests ROBOTEST_FILTER="WinscopeTraceTest"
Change-Id: Ie37790c7479f6b9064afd7fbcee7fd6f712d1a75