charger: android_get_control_file on last_kmsg

Call android_get_control_file on last_kmsg files if the file
descriptor is provided by init.

Also, uses base::Read(File|Fd)ToString functions to read the
files (because load_file doesn't support fd arguments).

Test: charger mode
Test: manual kernel panic, then start charger; seen last kmsg.

Bug: 129138950
Change-Id: Idd3376e349f29586a1e66faab2c0f1bf73e0eda5