Move PaletteReportLockContention to a new API version.

It was added in https://r.android.com/1801129, which means it went into
T and hence isn't in the first version of the Palette API.

Also add some comments to separate the API versions, and align
libartpalette.map.txt with the one in system/libartpalette after
https://r.android.com/2164906.

Test: m
Bug: 196334695
Change-Id: I68579fe8a64022d8ee7c828609240e4e3c29b32d