It seems there might have been a misunderstanding, possibly due to using a translator. My question wasn't about whether any Android app can use Amber.
Rather, I was asking: if a signer complies with NIP-55, can it be used for signing by a client, regardless of the specific signer implementation? This is similar to how client developers can request signing from any NIP-07 compliant signer (be it nos2x or Alby) without needing to specifically target or worry about which particular one it is.