Expose a BaseActivity methods to work around compiler/desugar bug.

This is a follow-up to commit 5d19b09512dd7a1bccce3142b97589d89ad68e56
which made a corresponding change in SetupWizard.

OpenJDK 9's LambdaMetaFactory's behavior differs from OpenJDK 8's when
a lambda or method reference (this::methodname) references a supertype's
protected method: OpenJDK 9 will generate a invokevirtual call to the
(inherited) child method whereas OpenJDK 8 generates an invokevirtual
call to the parent method.

If the child class is in a different package, this call fails in the
desugared lambda class (which lives in the child package) because the
lambda class is a non-subclass in the child's package and therefore
doesn't have access to the parent's protected method. Surprisingly,
this invocation succeeds at OpenJDK 9 runtime even when invoking java
-Xverify.

This behavior change was likely introduced upstream in [1] but the
corresponding bug [2] doesn't seem to exist. It is therefore not clear
whether this is a bug in LambdaMetaFactory, desugar, or an integration
bug between desugar and LambdaMetaFactory (perhaps a misunderstanding
of the semantics of the "owner" attribute).

Subclasses refer to protected BaseActivity methods via "this::popDir"
and "this::focusSidebar". This CL adds overrides to those methods
in the subclass. This makes the method available to other classes in
the same package (such as the desugared method reference/lambda),
fixing compilation.

[1] http://hg.openjdk.java.net/jdk9/dev/jdk/rev/a3b3c7b6464d
[2] https://bugs.openjdk.java.net/browse/JDK-8175806

Bug: 62218600
Test: make checkbuild (with OpenJDK 9 toolchain)
Change-Id: I7353dff1083c625cbde7ef1348c24e2848e6c0d2