Stop using C++ streams.

libvintf used to use std::ifstream to read files but incorrectly
assumed that a non-zero value of errno implied failure. Because the
implementation retries reads that fail with EAGAIN, this isn't true:
a successful operation could still result in errno being set to EAGAIN.

Use ReadFileToString from libbase which gives a clear boolean
success/failure signal.

Test: stress test
Bug: 156701275
Change-Id: Ieedba32f61a76ffcf96f7c06d89df70b5f94813b
1 file changed