Use CompOsKeyService as compsvc factory.

Define a Signer trait to encapsulate what we need to do to sign a
digest.

Modify compsvc to hold a signer.

Modify CompOsKeyService to be able to take in a keyblob and produce a
signer, then return a compsvc instance holding that signer.

This doesn't yet do anything with the signer. Eventually we will want
to use it to generate signatures on output artifacts.

Bug: 194267113
Test: atest ComposHostTestCases (with testOdrefesh un-ignored)
Change-Id: I72aead0280914987f7c8d1721c1e12ee0fad1af4
diff --git a/compos/Android.bp b/compos/Android.bp
index ad99d51..2c864e7 100644
--- a/compos/Android.bp
+++ b/compos/Android.bp
@@ -80,6 +80,7 @@
         "libbinder_rpc_unstable_bindgen",
         "libclap",
         "liblog_rust",
+        "libminijail_rust",
         "libring",
         "libscopeguard",
     ],