Uses main thread for ContentCaptureManager handler.

Initially it was creating a new thread, which infers in a ~0.3ms cost in the
initial activity creation. Examples:

Before:
com.android.settings: 403.542us
com.google.android.apps.nexuslauncher: 13147.397us
android.contentcaptureservice.cts: 481.979us

After:
com.android.settings: 25.677us
com.google.android.apps.nexuslauncher: 45.0us
android.contentcaptureservice.cts: 56.041us

Test: manually System.out check above
Test: atest CtsContentCaptureServiceTestCases

Fixes: 121039624

Change-Id: I4e7d90c4556467612d8b914fb3d3a5bc05a852bd