Move some definitions into anonymous namespaces

To prevent ODR violations, move some of the definitions into anonymous
namespaces.
Specifically, this solves the issue of multiple definitions for
FakeWindowHandle, which was triggering the leak sanitizer check when
'sanitize: address' was set to true.

Bug: 284203306
Test: m inputflinger_tests && $ANDROID_HOST_OUT/nativetest64/inputflinger_tests/inputflinger_tests
Change-Id: I106b2b547cf465df15e317a9b7478dcc77467a46