libbinder: avoid global mutex every binder call

We can't get rid of a mutex (or similar) for the rarer
ProcessState::selfOrNull access, but for ProcessState::self() accesses,
which happen at least once every binder call, we can use the very fast
once flags.

Bug: 161336582
Test: boots
Change-Id: I66fee6be0260f59f80a85328228776de7e68c7fb