Build: add hidden method to convert major.minor int to human readable string

Add a method to convert an int that represents a major.minor SDK version
(e.g. SDK_INT_FULL) into a human readable string (e.g. "43.21"). This is
intended only for debugging and error messages.

Bug: 377689343
Test: atest 'FrameworksCoreTests:android.os.BuildTest'
Flag: EXEMPT unused flag, call sites will be flagged when added
Change-Id: I2df5bfdee6e319b5ba1478032797aee23e941ae6