Remove redundant calls of MaybeMountProcFS and refactor it.

Remove redundant calls of MaybeMountProcFS and refactor it.

It seems that we have lots of redundant MaybeMountProcFS, in particular
each mapped sanitized shared library calls it from AsanInitializer constructor.
Remove redudant MaybeMountProcFS calls from AsanInitializer and __asan_init.

Also, it seems that under qemu-aarch64 opening /proc/self/maps for reading
might be very expensive, so replace OpenFile("/proc/self/maps", RdOnly) with
FileExists("/proc/self/maps") in MaybeMountProcFS to avoid expensive calls.

Change-Id: I8ba28dbbc102be519fa2b284520509f601ec5cf7
Signed-off-by: Maxim Ostapenko <m.ostapenko@samsung.com>