Require package:jni >= 0.7.1 to fix macOS build

[brianquinlan]

• package:jni 0.7.0 required jni headers to be present, which they may not be on macOS. This broke the macOS build for any flutter app depending on package:cronet_http, even though package:cronet_http is not used on macOS

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[HosseinYousefi]

LGTM!