cronet tests: Do not run integration tests on R

CtsNetHttpTestCases were missing a DevSdkIgnoreRule after updating the
min_sdk version to R+ (which made it installable on R where the cronet
classes are not in the bootclasspath.)

Test: atest CtsNetHttpTestCases
Bug: 259632936
Change-Id: I706ff92376227c1759fd6e542066fcff3a7cf052