Clarify PackageManager.hasSigningCertificate docs.

Address API review by describing relationship between the
PackageManager.hasSigningCertificate() methods and the PackageInfo
GET_SIGNING_CERTIFICATES method, as well as differentiating the
UID documentation from the package-name based one.

Bug: 74831566
Test: None, doc change.
Change-Id: I11c556325f9b2efbc2e5e1cf896b9c58db092ae8
1 file changed