Support pending CompOS keys.

When CompOS generates new artifacts it may also need to generate a new
keypair. Modify odsign to account for this. It needs to start up
CompOS to find out if the pending key is now the correct one; if so it
replaces the existing key, otherwise we keep the current key (if there
is one).

The pending keys are stored in /data/misc/apexdata/com.android.compos
since they are created & written by CompOS.

Fixed another RsaPublicKey bug.

Bug: 190166662
Test: Manual: No keys at all, no pending key, valid pending key, invalid pending key.
Change-Id: I17871b1e59380d037d71e8065f4cad699374ecaf
3 files changed