libprocessgroup: Make GetProfile() and GetAttribute() more efficient

Do not force callers to create an std::string object. This patch
implements the following advice from go/totw/1: "Google’s preferred
option for accepting such string parameters is through a string_view."
Use std::less<> as comparison type to prevent that std::string_view
objects have to be converted into std::string objects for lookups in
std::map<>.

Bug: 213617178
Change-Id: I08125a02220a8c003d9202a7e177be776c3b9829
Signed-off-by: Bart Van Assche <bvanassche@google.com>