Force the compilation reason of otapreopt to be "ab-ota".

The legacy code in the package manager uses "first-boot" as the
compilation reason for otapreopt when it wants to query
"pm.dexopt.first-boot" for the compiler filter. This causes confusion.

Bug: 307903002
Change-Id: I73c1178684f9056dc2d87e6f8d0379a64ac387c3
Test: Presubmit