Add unit test for Trampoline.

This adds the test to FrameworksServicesTests test suite, which can be
run with the following command:

make FrameworksServicesTests -j &&
adb install -r ${OUT}/data/app/FrameworksServicesTests/FrameworksServicesTests.apk &&
adb shell am instrument -w -e package com.android.server.backup \
com.android.frameworks.servicestests/android.support.test.runner.AndroidJUnitRunner

Bug: 37520021

Test: manually ran the test

Change-Id: I3f0dc50b0eb2ff2bc9360c68714c27e82b820897