Clean up some global variables in common.h

Some global variables are only used for recovery.cpp and
recovery_main.cpp, remove them from common.h and handle their usage
accordingly. Variables include:
static constexpr int kRecoveryApiVersion;
extern struct selabel_handle* sehandle;
extern RecoveryUI* ui;
extern bool has_cache;
bool is_ro_debuggable();

Test: unit tests pass, boot into recovery mode and run graphic tests
Change-Id: If83a005786c9b38412731da97aaf85af69a3b917
diff --git a/otautil/roots.cpp b/otautil/roots.cpp
index a778e05..4315517 100644
--- a/otautil/roots.cpp
+++ b/otautil/roots.cpp
@@ -51,6 +51,8 @@
 
 static Fstab fstab;
 
+constexpr const char* CACHE_ROOT = "/cache";
+
 void load_volume_table() {
   if (!ReadDefaultFstab(&fstab)) {
     LOG(ERROR) << "Failed to read default fstab";
@@ -275,3 +277,9 @@
   }
   return 0;
 }
+
+bool HasCache() {
+  CHECK(!fstab.empty());
+  static bool has_cache = volume_for_mount_point(CACHE_ROOT) != nullptr;
+  return has_cache;
+}