Add class docs for InputMonitor.

And clear Binder calling identity when monitoring gesture input. This is
a follow up for the review on Ic1e2cd6afea219d0e20b29e7150c9a42b75c7a67.

Bug: 123748692
Test: Manual
Change-Id: I72687fd8790927f76a2a56d4c5e80a4e9f89299c
2 files changed