Allow zipfuse to mount with fscontext and context

zipfuse now supports fscontext and context (but not defcontext yet).
Allow it to be mounted with zipfusefs as fscontext and system_file as
context.

Bug: 189165759
Bug: 188400186
Test: atest MicrodroidHostTestCases
Change-Id: I67a70b6d7975399ecfd0b4db87b50a8d25f89c4e