[android-recovery-zip] Add missing apk keys

I noticed that installing pmos with android-recovery, when running sudo apk update doesn't trust the alpine repository:

ERROR: https://nl.alpinelinux.org/alpine/edge/main: UNTRUSTED signature

With this PR the keys in /etc/apk/keys are also added.

It also compress the rootfs archive with gzip -f1 which reduces the time by ~30/40 seconds and the resulting zip grows only by 1-2 MB, for me it seems a good trade-off.