Add detailed reasons for dropping events in ViewRootImpl

Currently, ViewRootImpl drops events for several reasons. However, just
one reason is written to the log: "dropping event because the window
does not have focus".

But even if the window has focus, the event can be dropped.

Improve the logging by refactoring the "if" check inside ViewRootImpl

Test: atest ViewRootImplTest
Bug: 173675649
Change-Id: I2ba8270ba09dea63e6497bc830af4c704ada52ef