Remove LogCapture::WaitUntilLogContains async usage

After upgrading libc++, std::async is marked [[nodiscard]] in C++20
mode, which makes the usage of std::async in
LogCapture::WaitUntilLogContains a compiler error. The result of
std::async shouldn't be discarded, because its ~future destructor call
blocks until the function has completed, which defeats the purpose of
using std::async.

However, this code doesn't really need std::async, so remove the usage
of it.

Also change LogCapture::Find's parameter type to the ordinary
const-reference to reduce wasteful string copying.

Bug: 175635923
Test: atest bluetooth_test_gd_unit
Change-Id: I4c02250be1991a57156f1f8de2d78e7ad35f357d