Revert "Revert "Stop dropping caches now we have kernel fix""

This reverts commit 8ad0bef7b5.

Bug: 37231161
Test: Boot device with FBE enabled. ls /storage/emulated/0/Android
Unlock device. ls /storage/emulated/0/Android
1st will not be found. Second should be found.

Change-Id: I92c7ad0adaa7bd357e10661a47cc667ac0ff84b4
Merged-In: I92c7ad0adaa7bd357e10661a47cc667ac0ff84b4

Ext4Crypt.cpp

@@ -171,13 +171,6 @@ static bool install_key(const std::string& key, std::string* raw_ref) {
     LOG(DEBUG) << "Added key " << key_id << " (" << ref << ") to keyring " << device_keyring
                << " in process " << getpid();
 
-    // *TODO* Remove this code when kernel is fixed - see b/28373400
-    // Kernel preserves caches across a key insertion with ext4ice, which leads
-    // to contradictory dirents
-    if (!android::base::WriteStringToFile("3", "/proc/sys/vm/drop_caches")) {
-        PLOG(ERROR) << "Failed to drop_caches";
-    }
-
     return true;
 }