Add SecurityPolicy factory method for apps signed with the platform certificate

[mateusazis]

Addresses: #11238

[jdcormie]

We spoke a bit offline about how this seems just like a special case of oneOfSignatures.

[jdcormie]

Looks like the check failures above are real:

/grpc-java/binder/src/main/java/io/grpc/binder/SecurityPolicies.java:125: warning: [static] static variable should be qualified by type name, PackageManager, instead of by an expression
                        "android", packageManager.GET_SIGNING_CERTIFICATES);
                                                 ^
/grpc-java/binder/src/main/java/io/grpc/binder/SecurityPolicies.java:133: warning: [deprecation] signatures in PackageInfo has been deprecated
                      ImmutableList.copyOf(platformPackageInfo.signatures))
                                                              ^
error: warnings found and -Werror specified

[mateusazis]

Looks like the check failures above are real:

/grpc-java/binder/src/main/java/io/grpc/binder/SecurityPolicies.java:125: warning: [static] static variable should be qualified by type name, PackageManager, instead of by an expression
                        "android", packageManager.GET_SIGNING_CERTIFICATES);
                                                 ^
/grpc-java/binder/src/main/java/io/grpc/binder/SecurityPolicies.java:133: warning: [deprecation] signatures in PackageInfo has been deprecated
                      ImmutableList.copyOf(platformPackageInfo.signatures))
                                                              ^
error: warnings found and -Werror specified

The first warning is addressed.

For the second one, I made the code more compliant with the new post SDK 27 APIs (copied a bit from checkPackageSignature). Let me know if it works now or if we need a Lint suppression.