Use new cargo2android no-std flag rather than extra manual block. am: 2112f9de2a

Original change: https://android-review.googlesource.com/c/platform/external/rust/crates/serde/+/2583343

Change-Id: Iac0757900468592a7da2dfc928c7c3c0ca2de42d
Signed-off-by: Automerger Merge Worker <[email protected]>