printflags: introduce device tool to print feature flags

Metadata about all feature flags used on device are located in
/<partition>/etc/aconfig_flags.pb. Add a new tool, printflags, to read
and pretty-print these files.

printflags is only intended for debugging purposes.

Bug: 301547297
Test: adb shell printflags
Change-Id: I0a3277fecfc8a60eea0aa6bf362a25a311360b71