Installd: Change app image name computation

Refactor computation to take A/B OTAs into account.

Fix a const issue.

Bug: 25612095
Change-Id: I5cf79def532d1eeec4b35bb80014376959083a83
diff --git a/cmds/installd/otapreopt.cpp b/cmds/installd/otapreopt.cpp
index 05e4cfa..3775709 100644
--- a/cmds/installd/otapreopt.cpp
+++ b/cmds/installd/otapreopt.cpp
@@ -491,7 +491,6 @@
 ////////////////////////
 
 int get_property(const char *key, char *value, const char *default_value) {
-    // TODO: Replace with system-properties map.
     return gOps.GetProperty(key, value, default_value);
 }
 
@@ -499,9 +498,8 @@
 bool calculate_oat_file_path(char path[PKG_PATH_MAX], const char *oat_dir,
                              const char *apk_path,
                              const char *instruction_set) {
-    // TODO: Insert B directory.
-    char *file_name_start;
-    char *file_name_end;
+    const char *file_name_start;
+    const char *file_name_end;
 
     file_name_start = strrchr(apk_path, '/');
     if (file_name_start == nullptr) {