An efficient proc file reader

A less resource hogging reader for reading proc files in string
format. It is singleton and thread-safe, so all downstream clients
share the same instance to avoid duplicate allocation. It reads
the entire proc file all at once (to mimimize the impact to the
kernel) into a reusable char[] buffer (to prevent frequent GC). A
read lock is automatically held for the client when a valid file
iterator is returned. Client MUST call close() to unlock it or take
advantage of try-with-resources.

The reader keeps an error counter. It rejects further requests if it
has accumulated 5 errors, to prevent log spam. The reader also has
a 500ms cache, which can be bypassed with a parameter.

Bug: 111216804
Test: atest FrameworksCoreTests:com.android.internal.os.KernelCpuProcStringReaderTest
Change-Id: Ifa5213a5c7baf95d62f74486815030d9aa54ca28