Keep reading until end-of-file in reading procfs

As the pread(2) says "it is not an error for a successful call
to transfer fewer bytes than requested". The only way to verify
we're hitting the end-of-file is to keep reading from the file
until it returns 0. Here we're following this rule, in order to
make sure we read the whole procfs file to avoid the parser
errors. The drawback is that, now for every procfs read, it'll
have to hit the pread() at least twice. But if it has reached to
the end-of-file, the last read should return within microseconds.

Bug: 351917521
Flag: EXEMPT bugfix
Test: dumpsys cpuinfo
Change-Id: If6a14c4e8f4f1821628f1efb984f8064d4969a8c