Micro-optimize MergeCgroupToDescriptors()

Introduce a local variable for an expression that occurs twice. Use
string append instead of string concatenation because the former is more
efficient.

Bug: 213617178
Test: Compile-tested only.
Change-Id: I6e8b9d63b10accb220216dc484dffd18f5c54ce7
Signed-off-by: Bart Van Assche <bvanassche@google.com>