Add UnmountTreeWithPrefix util method.

Add a utility method to unmount all mountpoints
that start with a prefix.

Bug: 122905493
Test: manual
Change-Id: I11739e40e7849c1b4ca9e0b90c5c3f243691257a
diff --git a/Utils.h b/Utils.h
index 0b35a7b..48a57d9 100644
--- a/Utils.h
+++ b/Utils.h
@@ -127,7 +127,8 @@
 /* Checks if Android is running in QEMU */
 bool IsRunningInEmulator();
 
-status_t UnmountTree(const std::string& prefix);
+status_t UnmountTreeWithPrefix(const std::string& prefix);
+status_t UnmountTree(const std::string& mountPoint);
 
 status_t WaitForFile(const char* filename, std::chrono::nanoseconds timeout);