Update language to comply with Android's inclusive language guidance

See https://source.android.com/setup/contribute/respectful-code for reference

PerformAdb{Backup,Restore}Task handles backing up to and restoring from
a file. The backup data is encrypted with a randomly generated
encryption key. The encryption key is encrypted with a key that is
derived from a user supplied passphrase.

The key that was used to encrypt and decrypt the backup data used to be
called the "master key", I've renamed it to "encryption key" which
better describes its purpose.

Bug: 161896447
Test: `atest PerformAdbRestoreTaskTest`, `atest TarBackupReaderTest`
 and manually tested using adb with the following commands:
 `adb backup -all -apk -shared -f backup.ab` and
 `adb restore backup.ab`.
Change-Id: I8e6b22a06b2a583440839994f0626d370bdb2e19