Do not depend on android-tzdata if the clock feature is not enabled

All usages of `android-tzdata` types are within the `offset::local`
module, but such module is not built if the `clock` feature is not
enabled. Therefore, `android-tzdata` is effectively an unused dependency
when the `clock` feature is disabled.

To avoid downloading unnecessary crates on build time, let's make the
dependency on `android-tzdata` conditional on enabling the `clock`
feature, as it was done with e.g. `iana-time-zone`.

Cargo.toml

@@ -21,7 +21,7 @@ default = ["clock", "std", "oldtime", "wasmbind"]
 alloc = []
 libc = []
 std = []
-clock = ["std", "winapi", "iana-time-zone"]
+clock = ["std", "winapi", "iana-time-zone", "android-tzdata"]
 oldtime = ["time"]
 wasmbind = ["wasm-bindgen", "js-sys"]
 unstable-locales = ["pure-rust-locales", "alloc"]
@@ -50,7 +50,7 @@ winapi = { version = "0.3.0", features = ["std", "minwinbase", "minwindef", "tim
 iana-time-zone = { version = "0.1.45", optional = true, features = ["fallback"] }
 
 [target.'cfg(target_os = "android")'.dependencies]
-android-tzdata = "0.1.1"
+android-tzdata = { version = "0.1.1", optional = true }
 
 [dev-dependencies]
 serde_json = { version = "1" }