Remove untargeted injection fallback for targeted injection

This fallback was added because some tests performed targeted injection
into activities started by the shell. Since the known effected tests
were fixed in ag/17691131, we can try removing the fallback to see if
anything else breaks.

Bug: 228161340
Test: atest WindowFocusTests
Test: atest WindowInsetsControllerTests
Change-Id: I22d05ddd18d961e3c5d6568b53231d7888ea09fd