rust: Enable ELF TLS by default in platform Rust

Upstream currently uses pthread keys to emulate TLS on Android because
it historically did not have it, and is going to move to LLVM emulated
TLS, which is needed to target L. Since we know the platform is Q+, we
can just use the new features.

Test: Treehugger
Change-Id: I7c7747d89cba736377cfeb4080573a89b801e8fe