Use the 'override' keyword where appropriate

This patch implements the following advice from the Google C++ Style
Guide: "Explicitly annotate overrides of virtual functions or virtual
destructors with exactly one of an override or (less frequently) final
specifier. Do not use virtual when declaring an override. Rationale: A
function or destructor marked override or final that is not an override
of a base class virtual function will not compile, and this helps catch
common errors. The specifiers serve as documentation; if no specifier is
present, the reader has to check all ancestors of the class in question
to determine if the function or destructor is virtual or not."

Bug: 213617178
Test: Booted Android in Cuttlefish.
Change-Id: Iabe7ecd91a2c09a77922c60ff4a00314da509d4a
Signed-off-by: Bart Van Assche <bvanassche@google.com>