Use Native Serial Service for file operations.

Replace serial file operations previouly implemented as a part of
Serial system service by invocations of Native Serial Service,
so that we don't have to give SELinux file permissions to all
system services.

Bug: 415125368
Bug: 428744191
Flag: android.hardware.serial.flags.enable_wired_serial_api
Test: atest SerialTests
Change-Id: Id3ebf587879c77da4d1184417e08e7eec59ad3f7