Micro-optimize CgroupMap::ActivateControllers()

The C++ string concatenation operator is inefficient because it always
allocates a new string, even in cases where std::string::apppend() does
not allocate a new string. Hence this patch that replaces string
concatenation with a call to std::string::append().

Bug: 213617178
Test: Booted Android in Cuttlefish.
Change-Id: I79bdb89e957d3cfb40e48ef00c3e5576b4f533a5
Signed-off-by: Bart Van Assche <bvanassche@google.com>