Make the `partition=` tag optional.

Since we might use ToT release tools to sign a package
generated by an older build, we make the new
`partition=` tag optional. This also means we need to be
careful to use non-greedy regex matching.

Bug: 153133823
Test: python3 -m unittest
Test: input with and without the new `partition=` tag
Test: new test_ReadApkCerts_WithWithoutOptionalFields
Change-Id: Ic57efd34e745ad302ae17150c6f2318f0b4524cb
Merged-In: Ic57efd34e745ad302ae17150c6f2318f0b4524cb