inputflinger_tests: use sp w/ RefBase

make_unique is dangerous with this object, since implicit conversions to
sp would lead to double ownership

Bug: 232557259
Test: atest inputflinger_tests
Change-Id: I538123e20326ecad68b4a4b0bb820f50d3377c34
Merged-In: I2f469fd268472c7e78d36812353cff5c52a90163