Avoid ByteBuffer#asReadOnlyBuffer()

[overheadhunter]

Except for some cases where a ReadOnlyBuffer is used in a very limited scope, we don't want to use them in conjunction with Ciphers and Macs for the following reason:

A readonly buffer doesn't allow direct access to its byte[], regardless of whether its on-heap or not. However, Ciphers are more efficient when ByteBuffer#hasArray() returns true (as can be seen e.g. in CipherSpi) as they don't need to copy bytes from the buffers to some internal byte[].

To avoid allocations of byte[], we will therefore use .duplicate() instead of .asReadOnlyBuffer(), even if we don't need write access to said buffers.

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

100.0% Coverage
11.8% Duplication

[SailReal]

LGTM 👍 and tested it using our new min version (Android 8.0) with vault format 7/8

* create/open a vault
* create/open a file/folder