[ECM] Strip colons from sha256-cert-digest

Strip colons from the "sha256-cert-digest" XML attribute of the
recently introduced <enhanced-confirmation-trusted-package> and
<enhanced-confirmation-trusted-installer> SysConfig XML elements,
in effect making colons optional.

The reason for this is that, while we've documented this attribute to
be of the form:

    sha256-cert-digest="E9:7A:BC:2C:D1:..."

...the parsing code, android.content.pm.Signature, expects zero colons.

Fix: 323603510
Test: atest SystemConfigTest
Change-Id: Ida7a49ece110fa66de65fbe966e180a40f974a5e