vold: const-ify some of the API

We make some of the C++ member functions 'const', to reflect how
they (don't) affect the state of the class.

Test: TreeHugger
Change-Id: Iec1c2801bfe721e2741406ed1ac0ef95662840a6
diff --git a/model/VolumeBase.cpp b/model/VolumeBase.cpp
index 29bc32c..a9c7fa3 100644
--- a/model/VolumeBase.cpp
+++ b/model/VolumeBase.cpp
@@ -153,7 +153,7 @@
     return OK;
 }
 
-android::sp<android::os::IVoldListener> VolumeBase::getListener() {
+android::sp<android::os::IVoldListener> VolumeBase::getListener() const {
     if (mSilent) {
         return nullptr;
     } else {