Allow 15 byte headtracker SDU for early implementations

Early drafts of DSA 2.0 specification had 15 byte allocation for
headtracker data over LE-ISO. This was later changed to 13 bytes.
However some early adopters of DSA 2.0 may expect an allocation of 15
bytes for headtracker data.
Early drafts of DSA 2.0 specification also didn't have metadata for
Android Headtracker Codec. This can be used to distinguish between
earbuds which support early and those which support approved DSA 2.0
specification.

Test: mmm packages/modules/Bluetooth
Test: Manual | Test DSA 2.0 with all supporting earbuds
Flag: com.android.bluetooth.flags.headtracker_sdu_size
Bug: 349901879
Bug: 350090733
Change-Id: If70046cd801a95d3b628450ba223ffe5010ffc3e