Zero memory used for encryuption keys.

std::vector with custom zeroing allocator is used instead of
std::string for data that can contain encryption keys.

Bug: 64201177
Test: manually created a managed profile, changed it's credentials
Test: manually upgraded a phone with profile from O to MR1.
Change-Id: Ic31877049f69eba9f8ea64fd99acaaca5a01d3dd
diff --git a/Android.mk b/Android.mk
index 9dba651..d0b199d 100644
--- a/Android.mk
+++ b/Android.mk
@@ -27,6 +27,7 @@
 	MoveTask.cpp \
 	Benchmark.cpp \
 	TrimTask.cpp \
+	KeyBuffer.cpp \
 	Keymaster.cpp \
 	KeyStorage.cpp \
 	KeyUtil.cpp \