[rand] Expose rand_core, rand_os, and rand_xorshift crates to fuchsia

The following libraries use just a subset of the rand library:

* garnet/bin/mediasession uses `rand::random`
* garnet/bin/recovery_netstack uses `rand::{SeedableRng,XorShiftRng}`
* wlan-rsn uses `rand::OsRng`

In order to make this code easier to audit, we can swap these uses
with `rand_core`, `rand_os`, and `rand_xorshift`.

Change-Id: Ic0f74884b2cb1dbb791c6467007c05a3c76c830f
diff --git a/rustc_deps/Cargo.lock b/rustc_deps/Cargo.lock
index 58f1b4a..071853b 100644
--- a/rustc_deps/Cargo.lock
+++ b/rustc_deps/Cargo.lock
@@ -526,6 +526,9 @@
  "proptest 0.8.7 (registry+https://github.com/rust-lang/crates.io-index)",
  "quote 0.6.10 (registry+https://github.com/rust-lang/crates.io-index)",
  "rand 0.6.5 (registry+https://github.com/rust-lang/crates.io-index)",
+ "rand_core 0.4.0 (registry+https://github.com/rust-lang/crates.io-index)",
+ "rand_os 0.1.2 (registry+https://github.com/rust-lang/crates.io-index)",
+ "rand_xorshift 0.1.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "regex 1.1.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "ring 0.14.5 (registry+https://github.com/rust-lang/crates.io-index)",
  "rouille 3.0.0 (registry+https://github.com/rust-lang/crates.io-index)",
diff --git a/rustc_deps/Cargo.toml b/rustc_deps/Cargo.toml
index 3666b0c..f47b591 100644
--- a/rustc_deps/Cargo.toml
+++ b/rustc_deps/Cargo.toml
@@ -51,6 +51,9 @@
 pest_derive = "2.1"
 proptest = "0.8.7"
 rand = "0.6"
+rand_core = "0.4.0"
+rand_os = "0.1.2"
+rand_xorshift = "0.1.1"
 ring = "0.14"
 regex = "1.0.6"
 rouille = "3.0.0"