power: fix TryDifferentGovernors test

Switch to std::string and libbase to simplify the test and avoid
string manipulation bugs

Bug: 154070740
Bug: 149044096
Test: atest VtsHalPowerV1_0TargetTest
Change-Id: Ic80ca462cfeb650edcdc6420bf1b3190aa14dbe4
Signed-off-by: Connor O'Brien <connoro@google.com>