Change APK buffer from &mut [u8] to &[u8]

[dairlair]

This change makes sign_apk_buffer API cleaner and safer by removing the need for &mut [u8] input. The signing pipeline now operates on immutable input data (&[u8]), improving API ergonomics and eliminating misleading mutability requirements.

In usage under production load, consumers of the library were required to create a full mutable copy of the entire APK buffer in order to satisfy the previous &mut [u8] API constraint. Now it is not needed.

It also changes compute_first_level_hashes function.

We now only allocate memory for Chunk 4 (EOCD section of the ZIP), which is the only smallest part that requires modification. All other ZIP chunks are processed as zero-copy slices of the input buffer.

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[dairlair]

@adamsoutar one small change here: the function signature was updated from

pub fn sign_apk_buffer(apk_buf: &mut [u8], keys: &Keys) -> Result<Vec>

to

pub fn sign_apk_buffer(apk_buf: &[u8], keys: &Keys) -> Result<Vec>

Since the buffer is not mutated, using an immutable slice is more idiomatic in Rust and improves safety by preventing unintended modifications.